[build-script] Add --parallel to supress output on success

[benlangmuir]

It's slightly faster and it helps supress the test output when tests
succeed (which avoids false positives in Jenkins' error regexes, for
example the error from "let pack" in a test).

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please test Linux

[benlangmuir]

Looks like testCloseRace hangs in waitForExpectations(timeout: 10) on Linux when running with --parallel.

It's polling file descriptors, so probably double close on the FD or something.

* thread #1, name = 'SourceKitLSPPac', stop reason = signal SIGSTOP
    frame #0: 0x00007f389125ccf6 libc.so.6`ppoll + 166
    frame #1: 0x00007f3893b8e36d libFoundation.so`__CFRunLoopServiceFileDescriptors + 285
    frame #2: 0x00007f3893b8a2d1 libFoundation.so`__CFRunLoopRun + 1153
    frame #3: 0x00007f3893b89bc9 libFoundation.so`CFRunLoopRunSpecific + 473
    frame #4: 0x00007f3893ae22c2 libFoundation.so`Foundation.RunLoop.run(mode: Foundation.RunLoop.Mode, before: Foundation.Date) -> Swift.Bool + 210
    frame #5: 0x00007f3893e9be56 libXCTest.so`XCTest.XCTWaiter.wait(for: Swift.Array<XCTest.XCTestExpectation>, timeout: Swift.Double, enforceOrder: Swift.Bool, file: Swift.StaticString, line: Swift.Int) -> XCTest.XCTWaiter.Result + 534
    frame #6: 0x00007f3893ea1218 libXCTest.so`XCTest.XCTestCase.waitForExpectations(timeout: Swift.Double, file: Swift.StaticString, line: Swift.Int, handler: Swift.Optional<(Swift.Optional<Swift.Error>) -> ()>) -> () + 344
  * frame #7: 0x0000560ed8e6deab SourceKitLSPPackageTests.xctest`ConnectionTests.testCloseRace(self=<invalid> @ 0x00007ffff7c77ae0) at ConnectionTests.swift:248:7 [opt]
    frame #8: 0x0000560ed8e75e61 SourceKitLSPPackageTests.xctest`partial apply for thunk for @escaping @callee_guaranteed () -> () [inlined] reabstraction thunk helper from @escaping @callee_guaranteed () -> () to @escaping @callee_guaranteed () -> (@out ()) at <compiler-generated>:0 [opt]
    frame #9: 0x0000560ed8e75e5e SourceKitLSPPackageTests.xctest`partial apply for thunk for @escaping @callee_guaranteed () -> () at <compiler-generated>:0 [opt]
    frame #10: 0x0000560ed8e763c9 SourceKitLSPPackageTests.xctest`reabstraction thunk helper from @escaping @callee_guaranteed () -> () to @escaping @callee_guaranteed () -> (@out ())partial apply forwarder with unmangled suffix ".60" + 9
    frame #11: 0x00007f3893e8bc01 libXCTest.so`partial apply forwarder for reabstraction thunk helper from @escaping @callee_guaranteed () -> (@out ()) to @escaping @callee_guaranteed () -> () + 17
    frame #12: 0x00007f3893e8bc6c libXCTest.so`partial apply forwarder for closure #1 (XCTest.XCTestCase) throws -> () in XCTest.(test in _3BE257A46ADB477C7BF2D39968B39F9D)<A where A: XCTest.XCTestCase>((A) -> () throws -> ()) -> (XCTest.XCTestCase) throws -> () + 60
    frame #13: 0x00007f3893e8bbe4 libXCTest.so`partial apply forwarder for reabstraction thunk helper from @escaping @callee_guaranteed (@guaranteed XCTest.XCTestCase) -> (@error @owned Swift.Error) to @escaping @callee_guaranteed (@in_guaranteed XCTest.XCTestCase) -> (@out (), @error @owned Swift.Error) + 20
    frame #14: 0x00007f3893e7c847 libXCTest.so`partial apply forwarder for reabstraction thunk helper from @escaping @callee_guaranteed (@in_guaranteed XCTest.XCTestCase) -> (@out (), @error @owned Swift.Error) to @escaping @callee_guaranteed (@guaranteed XCTest.XCTestCase) -> (@error @owned Swift.Error) + 39
    frame #15: 0x00007f3893e8a597 libXCTest.so`XCTest.XCTestCase.invokeTest() -> () + 55
    frame #16: 0x00007f3893e8a2d6 libXCTest.so`XCTest.XCTestCase.perform(XCTest.XCTestRun) -> () + 150
    frame #17: 0x00007f3893e8df56 libXCTest.so`XCTest.XCTest.run() -> () + 150
    frame #18: 0x00007f3893e8c1f0 libXCTest.so`XCTest.XCTestSuite.perform(XCTest.XCTestRun) -> () + 160
    frame #19: 0x00007f3893e8df56 libXCTest.so`XCTest.XCTest.run() -> () + 150
    frame #20: 0x00007f3893e8c1f0 libXCTest.so`XCTest.XCTestSuite.perform(XCTest.XCTestRun) -> () + 160
    frame #21: 0x00007f3893e8df56 libXCTest.so`XCTest.XCTest.run() -> () + 150
    frame #22: 0x00007f3893e88d4a libXCTest.so`XCTest.XCTMain(_: Swift.Array<(testCaseClass: XCTest.XCTestCase.Type, allTests: Swift.Array<(Swift.String, (XCTest.XCTestCase) throws -> ())>)>, arguments: Swift.Array<Swift.String>) -> Swift.Never + 1482
    frame #23: 0x00007f3893e88779 libXCTest.so`XCTest.XCTMain(Swift.Array<(testCaseClass: XCTest.XCTestCase.Type, allTests: Swift.Array<(Swift.String, (XCTest.XCTestCase) throws -> ())>)>) -> Swift.Never + 25
    frame #24: 0x0000560ed90466ed SourceKitLSPPackageTests.xctest`main at LinuxMain.swift:20:1 [opt]
    frame #25: 0x00007f3891169b97 libc.so.6`__libc_start_main + 231
    frame #26: 0x0000560ed8c0a26a SourceKitLSPPackageTests.xctest`_start + 42

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please test macOS