SPM Prep - Use XCTUnwrap instead of force-unwrapping for fixture paths in tests

[mokagio]

Description

This will make it easier to run the tests once the fixture files move around.

If a file is misplaced, the test will fail instead of crashing.

Operationally, failing is better than crashing because it generates a
list of all misplaced tests, so that they can be addressed in one go. The alternative would be a run tests, tests crash, fix individual file path loop for as many misplaced fixture files there are.

Unfortunately, the diff is extensive. However, with the exception of RemoteTestCase.swift which changed in isolation in 007e11a, the pattern for all the changes is the same:

From:

func testSomething() {
    stub(condition: {  ... }) { _ in
      let stubPath = OHPathForFile("fixture.json", type(of: self))
      return fixture(filePath: stubPath!, ...)
    }
    ...

To:

func testSomething() throws { // test method now throws because of try XCTUnwrap
    // stubPath defined outside stub closure because the closure does not throw
    let stubPath = try XCTUnwrap(OHPathForFile("fixture.json", type(of: self))) 
    stub(condition: {  ... }) { _ in
      return fixture(filePath: stubPath, ...) // direct access instead of force unwrap
    }
    ...

Testing Details

Green CI. This is only a unit tests implementation change.

Next up

Basically, cherry-pick the work from #738 but in a neat and reviewable order:

• Move files into Sources/ and Tests/
• Introduce Bundle helper that differentiates between SPM and CocoaPods installations
• Isolate core API objects in dedicated package
• Then, new ground, define protocol abstraction to work around the Objective-C / Swift inter-op issue

---

• Please check here if your pull request includes additional test coverage. — N.A.
• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary. — N.A.

[mokagio]

The header dictionary for content type JSON is repeated all over the place.

It could be DRYed in some form. I decided to keep it out because this PR is already noisy. Besides, the focus of this stream of work is to prepare for SPM. DRYing this definition would be useful, but doesn't bring us closer to that goal.

[mokagio]

Had to fail in this way, also returning an error because we're inside a closure and the file path is computed based on the callCounter value.