Add an umbrella import to the _TestingInternals module map file. (#431)

This PR removes the individual header declarations from the
`_TestingInternals` module map file and replaces them with a single
umbrella import. This fixes an existing workflow used by some of our
teammates.

### 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/_TestingInternals/include/module.modulemap

@@ -1,10 +1,14 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
 module _TestingInternals {
-  header "Defines.h"
-  header "Discovery.h"
-  header "GetSymbol.h"
-  header "Includes.h"
-  header "Stubs.h"
-  header "TestSupport.h"
-  header "WillThrow.h"
+  umbrella "."
   export *
 }