Remove @convention(thin) from exit test macro declarations. (#1153)

`@convention(thin)` is not fully plumbed through the compiler and should
not be in our API surface, but also because of the experimental value
capturing feature the closures may be thick anyway.

See also swiftlang/swift-evolution#2884.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Expectations/Expectation+Macro.swift

@@ -525,7 +525,7 @@ public macro expect(
   observing observedValues: [any PartialKeyPath<ExitTest.Result> & Sendable] = [],
   _ comment: @autoclosure () -> Comment? = nil,
   sourceLocation: SourceLocation = #_sourceLocation,
-  performing expression: @escaping @Sendable @convention(thin) () async throws -> Void
+  performing expression: @escaping @Sendable () async throws -> Void
 ) -> ExitTest.Result? = #externalMacro(module: "TestingMacros", type: "ExitTestExpectMacro")
 
 /// Check that an expression causes the process to terminate in a given fashion
@@ -572,7 +572,7 @@ public macro require(
   observing observedValues: [any PartialKeyPath<ExitTest.Result> & Sendable] = [],
   _ comment: @autoclosure () -> Comment? = nil,
   sourceLocation: SourceLocation = #_sourceLocation,
-  performing expression: @escaping @Sendable @convention(thin) () async throws -> Void
+  performing expression: @escaping @Sendable () async throws -> Void
 ) -> ExitTest.Result = #externalMacro(module: "TestingMacros", type: "ExitTestRequireMacro")
 
 /// Capture a sendable and codable value to pass to an exit test.

Sources/Testing/Expectations/ExpectationChecking+Macro.swift

@@ -1148,7 +1148,7 @@ public func __checkClosureCall(
   identifiedBy exitTestID: (UInt64, UInt64, UInt64, UInt64),
   processExitsWith expectedExitCondition: ExitTest.Condition,
   observing observedValues: [any PartialKeyPath<ExitTest.Result> & Sendable] = [],
-  performing _: @convention(thin) () -> Void,
+  performing _: @convention(c) () -> Void,
   expression: __Expression,
   comments: @autoclosure () -> [Comment],
   isRequired: Bool,
@@ -1181,7 +1181,7 @@ public func __checkClosureCall<each T>(
   encodingCapturedValues capturedValues: (repeat each T),
   processExitsWith expectedExitCondition: ExitTest.Condition,
   observing observedValues: [any PartialKeyPath<ExitTest.Result> & Sendable] = [],
-  performing _: @convention(thin) () -> Void,
+  performing _: @convention(c) () -> Void,
   expression: __Expression,
   comments: @autoclosure () -> [Comment],
   isRequired: Bool,