Work around a crash importing FoundationXML. (#786)

This PR works around a new crash importing FoundationXML See:

-
https://ci-external.swift.org/job/swift-testing-pull-request-windows/832/
- https://ci.swift.org/job/pr-swift-testing-linux-5.10/1319/console

Works around rdar://138761752.

### 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

Tests/TestingTests/EventRecorderTests.swift

@@ -15,7 +15,7 @@ import RegexBuilder
 #if canImport(Foundation)
 import Foundation
 #endif
-#if canImport(FoundationXML)
+#if SWT_FIXED_138761752 && canImport(FoundationXML)
 import FoundationXML
 #endif
 
@@ -299,7 +299,7 @@ struct EventRecorderTests {
   }
 #endif
 
-#if canImport(Foundation) || canImport(FoundationXML)
+#if (SWT_TARGET_OS_APPLE && canImport(Foundation)) || (SWT_FIXED_138761752 && canImport(FoundationXML))
   @Test(
     "JUnitXMLRecorder outputs valid XML",
     .bug("https://github.com/swiftlang/swift-testing/issues/254")