Avoid Regex API availability check when no filter or skip flags have been passed (#410)

Fix several test failures in `SwiftPMTests` when running on older Apple
OS versions before `Regex` was added, which is due to unnecessarily
checking whether that API is available. One example:

```text
✘ Test "No --filter or --skip argument" recorded an issue at SwiftPMTests.swift:40:4: Caught error: The `--filter' option is not supported on this OS version.
```

In tests which do not involve `--filter` or `--skip` arguments, this
check is unnecessary and can be avoided altogether.

### 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: stmontgomery

Diff for: Sources/Testing/EntryPoints/EntryPoint.swift

@@ -313,7 +313,9 @@ public func configurationForEntryPoint(from args: __CommandLineArguments_v0) thr
   // Filtering
   var filters = [Configuration.TestFilter]()
   func testFilter(forRegularExpressions regexes: [String]?, label: String, membership: Configuration.TestFilter.Membership) throws -> Configuration.TestFilter {
-    guard let regexes else {
+    guard let regexes, !regexes.isEmpty else {
+      // Return early if empty, even though the `reduce` logic below can handle
+      // this case, in order to avoid the `#available` guard.
       return .unfiltered
     }