github: Copy in the dependency DLLs into the bin dir in test-llvm

This avoids relying on finding these dependencies in $PATH while
running the tests. Some tests may set PATH="" while executing them,
which previously caused those tests to fail in this build setup.

The same is not an issue with MSVC builds with
-DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreadedDLL, because those
DLLs are also available in global system directories (including
DLLs such as msvcp140.dll and vcruntime140.dll).

.github/workflows/test-llvm.yml

@@ -87,6 +87,8 @@ jobs:
             -DCMAKE_CXX_COMPILER=clang++ `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja test-depends
+          cp c:\llvm-mingw\bin\libc++.dll bin
+          cp c:\llvm-mingw\bin\libunwind.dll bin
       - name: Run tests
         run: |
           cd llvm-project