_SwiftSyntaxGenericTestSupport uses key paths which are unavailable in Embedded Swift

## Description

When building a package that depends on swift-syntax with the Embedded Swift SDK, compilation fails because `_SwiftSyntaxGenericTestSupport` uses key path syntax which is not available in Embedded Swift.

## Error

```
error: cannot use key path in embedded Swift
|
18 | .map { $0.droppingLast(while: \.isWhitespace) }
| `- error: cannot use key path in embedded Swift
```

## Location

`Sources/_SwiftSyntaxGenericTestSupport/String+TrimmingTrailingWhitespace.swift:18`

## Suggested Fix

Replace key path with explicit closure:

```swift
// Before
.map { $0.droppingLast(while: \.isWhitespace) }

// After
.map { $0.droppingLast(while: { $0.isWhitespace }) }
```

## Environment

- Swift 6.2.3 (swift-6.2.3-RELEASE)
- SDK: swift-6.2.3-RELEASE_wasm-embedded
- Build command: `swift build --swift-sdk swift-6.2.3-RELEASE_wasm-embedded`

## Notes

This only affects test support code, not the core SwiftSyntax library. However, downstream packages that include swift-syntax as a dependency cannot build for Embedded Swift targets even if they don't use the test support module, because SwiftPM builds all targets.