Modify assertMacroExpansion() so that it only conditionally calls XCTFail().

This PR changes the signature of `assertMacroExpansion()` such that an alternative failure handler can be passed for use when e.g. swift-testing is being used.

Right now, the function calls `XCTAssertEqual()`, `XCTFail()`, etc. on the assumption that the caller is using XCTest. This assumption does not always hold and the dependency on XCTest may prevent a developer from writing effective tests leveraging `assertMacroExpansion()` while also using swift-testing or other testing libraries.

It is not possible for swift-syntax to include swift-testing as a dependency at this time, so the function cannot be amended to directly call the equivalent of `XCTFail()` from that library. Nor can the function check if the current process is running an XCTest-based test because XCTest does not expose API to determine if a test is running.

Until such time as swift-syntax can directly call into swift-testing, developers can do so when calling `assertMacroExpansion()` like so:

```swift
assertMacroExpansion(
  "@foo bar",
  "expanded_foo bar",
  macros: ...,
  ...
) { message, fileID, filePath, line, column in
  Issue.record(Comment(rawValue: message), fileID: fileID, filePath: String(describing: filePath), Int(line), Int(column))
}
```

An improved interface is a future direction.

Resolves #2400.

Author: grynspan