Set availability for inverted character class test

This feature depends on running with a Swift 5.7 stdlib, and fails
when that isn't available.

Author: natecook1000

Package.swift

@@ -93,7 +93,7 @@ let package = Package(
             name: "DocumentationTests",
             dependencies: ["_StringProcessing", "RegexBuilder"],
             swiftSettings: [
-                .unsafeFlags(["-Xfrontend", "-disable-availability-checking"]),
+                availabilityDefinition,
                 .unsafeFlags(["-enable-bare-slash-regex"]),
             ]),
 

Tests/DocumentationTests/RegexBuilderTests.swift

@@ -19,6 +19,7 @@ import RegexBuilder
 
 class RegexBuilderTests: XCTestCase {}
 
+@available(SwiftStdlib 5.7, *)
 extension RegexBuilderTests {
   func testCharacterClass_inverted() throws {
     let validCharacters = CharacterClass("a"..."z", .anyOf("-_"))

Tests/DocumentationTests/RegexTests.swift

@@ -18,6 +18,7 @@ import _StringProcessing
 
 class RegexTests: XCTestCase {}
 
+@available(SwiftStdlib 5.7, *)
 extension RegexTests {
   func testRegex() throws {
     // 'keyAndValue' is created using a regex literal
@@ -91,6 +92,7 @@ extension RegexTests {
   }
 }
 
+@available(SwiftStdlib 5.7, *)
 extension RegexTests {
   func testRegex_wholeMatchIn() throws {
     let digits = /[0-9]+/