Simplify the expansion of @Test by eliminating the isolated thunk.

[grynspan]

This PR removes one of the thunks we emit when expanding @Test. For example, given the following test function:

@Test func foo() {}

We currently emit, approximately:

@available(*, deprecated, message: "This function is an implementation detail of the testing library. Do not use it directly.")
@Sendable private  func $s12TestingTests3foo4TestfMp_17Z152ec26a56a4f672fMu_() async throws -> Void {
  @Sendable func $s12TestingTests3foo4TestfMp_7__localfMu_(_:isolated (any _Concurrency.Actor)?=Testing.__defaultSynchronousIsolationContext) async throws {
    _ = unsafe try await Testing.__requiringUnsafe(Testing.__requiringTry(Testing.__requiringAwait(foo())))
  }
  try await $s12TestingTests3foo4TestfMp_7__localfMu_()
}

@available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
@Sendable private  func $s12TestingTests3foo4TestfMp_25generator152ec26a56a4f672fMu_() async -> Testing.Test {
  return .__function(
  named: "foo()",
  in: nil as Swift.Never.Type?,
  xcTestCompatibleSelector: nil,
  traits: [],sourceBounds: Testing.__SourceBounds(__uncheckedLowerBound: Testing.SourceLocation(__uncheckedFileID: "TestingTests/ZipTests.swift", filePath: "/Volumes/Dev/Source/swift-testing-public/Tests/TestingTests/ZipTests.swift", line: 30, column: 2), upperBound: (30, 20)),
  parameters: [],
  testFunction: $s12TestingTests3foo4TestfMp_17Z152ec26a56a4f672fMu_
)
}

@section("__DATA_CONST,__swift5_tests")
@used
@available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
private nonisolated  let $s12TestingTests3foo4TestfMp_33testContentRecord152ec26a56a4f672fMu_: Testing.__TestContentRecord = (
  0x74657374, /* 'test' */
  0,
  { outValue, type, _, _ in
    Testing.Test.__store($s12TestingTests3foo4TestfMp_25generator152ec26a56a4f672fMu_, into: outValue, asTypeAt: type)
  },
  0,
  0
)

Note that the first thunk function has a nested thunk function that serves only to impose actor isolation on the actual test function. This change replaces that thunk with nonisolated(nonsending) in appropriate locations throughout the macro target and library target. After this change, we emit a simpler expansion:

@available(*, deprecated, message: "This function is an implementation detail of the testing library. Do not use it directly.")
@Sendable private nonisolated(nonsending)  func $s12TestingTests3foo4TestfMp_17Z152ec26a56a4f672fMu_() async throws -> Void {
  _ = unsafe try await Testing.__requiringUnsafe(Testing.__requiringTry(Testing.__requiringAwait(foo())))
}

@available(*, deprecated, message: "This function is an implementation detail of the testing library. Do not use it directly.")
@Sendable private nonisolated(nonsending)  func $s12TestingTests3foo4TestfMp_25generator152ec26a56a4f672fMu_() async -> Testing.Test {
  return .__function(
  named: "foo()",
  in: nil as Swift.Never.Type?,
  xcTestCompatibleSelector: nil,
  traits: [],sourceBounds: Testing.__SourceBounds(__uncheckedLowerBound: Testing.SourceLocation(__uncheckedFileID: "TestingTests/ZipTests.swift", filePath: "/Volumes/Dev/Source/swift-testing-public/Tests/TestingTests/ZipTests.swift", line: 30, column: 2), upperBound: (30, 20)),
  parameters: [],
  testFunction: $s12TestingTests3foo4TestfMp_17Z152ec26a56a4f672fMu_
)
}

@section("__DATA_CONST,__swift5_tests")
@used
@available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
private nonisolated  let $s12TestingTests3foo4TestfMp_33testContentRecord152ec26a56a4f672fMu_: Testing.__TestContentRecord = (
  0x74657374, /* 'test' */
  0,
  { outValue, type, _, _ in
  Testing.Test.__store($s12TestingTests3foo4TestfMp_25generator152ec26a56a4f672fMu_, into: outValue, asTypeAt: type)
},
  0,
  0
)

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[harlanhaskins]

nitpick: this seems unnecessary

[harlanhaskins]

It's quite nice to see language features obviating the need for workarounds in our code. Very nice

[grynspan]

Compiler angry!