Fix a symbol marked @_spi that's needed for exit test value capturing. (#1335)

I missed removing an `@_spi` attribute when enabling exit test value
capturing for
[ST-0012](https://github.com/swiftlang/swift-evolution/blob/main/proposals/testing/0012-exit-test-value-capturing.md).

### 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/ExpectationChecking+Macro.swift

@@ -1175,7 +1175,6 @@ public func __checkClosureCall(
 ///
 /// - Warning: This function is used to implement the `#expect()` and
 ///   `#require()` macros. Do not call it directly.
-@_spi(Experimental)
 public func __checkClosureCall<each T>(
   identifiedBy exitTestID: (UInt64, UInt64, UInt64, UInt64),
   encodingCapturedValues capturedValues: (repeat each T),

Tests/TestingTests/ExitTestTests.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-@testable @_spi(Experimental) @_spi(ForToolsIntegrationOnly) import Testing
+@testable @_spi(ForToolsIntegrationOnly) import Testing
 private import _TestingInternals
 
 #if !SWT_NO_EXIT_TESTS