GHA: correct swift-testing macro provider

The plugin name is `TestingMacros.dll` not `SwiftTesting.dll`. Correct
the spelling for the build.

Author: compnerd

.github/workflows/swift-toolchain.yml

@@ -2138,7 +2138,7 @@ jobs:
                 -G Ninja `
                 -S ${{ github.workspace }}/SourceCache/swift-testing `
                 -D SwiftSyntax_DIR=${{ github.workspace }}/BinaryCache/swift-syntax/cmake/modules `
-                -D SwiftTesting_MACRO=${{ github.workspace }}/BinaryCache/Library/Developer/Toolchains/unknown-Asserts-development.xctoolchain/usr/bin/SwiftTesting.dll
+                -D SwiftTesting_MACRO=${{ github.workspace }}/BinaryCache/Library/Developer/Toolchains/unknown-Asserts-development.xctoolchain/usr/bin/TestingMacros.dll
       - name: Build Testing
         run: |
           cmake --build ${{ github.workspace }}/BinaryCache/testing