[Validation] Make Validation tests more robust (microsoft#7848)

This PR just addresses some final concerns in
microsoft#7749
It also adds a missing REQUIRES line that should've been added to a test
that merged into the repo while the previous PR was in development.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: bob80905

lib/DxcSupport/dxcapi.extval.cpp

@@ -319,7 +319,7 @@ class ExternalValidationCompiler : public IDxcCompiler2, public IDxcCompiler3 {
   template <typename T> CComPtr<T> cast() const {
     CComPtr<T> Result;
     if (Compiler)
-      Compiler->QueryInterface(__uuidof(T), reinterpret_cast<void **>(&Result));
+      Compiler.QueryInterface(&Result);
     assert(Result);
     return Result;
   }

tools/clang/test/CodeGenDXIL/hlsl/intrinsics/asuint-constant-eval.hlsl

@@ -1,3 +1,4 @@
+// REQUIRES: dxil-1-9
 // RUN: %dxc /T ps_6_9 -fcgl %s | FileCheck %s
 
 // Compiling this HLSL would fail this assertion in IntExprEvaluator::Success:

tools/clang/unittests/HLSL/ValidationTest.cpp

@@ -4274,8 +4274,8 @@ TEST_F(ValidationTest, UnitTestExtValidationSupport) {
   VERIFY_ARE_EQUAL_STR(BogusPath.c_str(), "bogus");
   VERIFY_IS_TRUE(ExtSupportBogus.dxilDllFailedToLoad());
 
-  // 3. Test with a valid path to this file in the environment variable
-  std::filesystem::path p = std::filesystem::absolute(__FILE__);
+  // 3. Test with a valid path to a file in the environment variable
+  std::filesystem::path p = hlsl_test::GetPathToHlslDataFile(L"lit.local.cfg");
   SetEnvVarW(L"DXC_DXIL_DLL_PATH", p.wstring());
 
   if (!ExtSupportValidNonDLLPath.IsEnabled()) {
@@ -4287,7 +4287,10 @@ TEST_F(ValidationTest, UnitTestExtValidationSupport) {
   // validate that ExtSupportValidNonDLLPath was able to capture the environment
   // variable's value, and that loading the valid non-dll path was unsuccessful
   std::string ValidNonDLLPath = ExtSupportValidNonDLLPath.getDxilDllPath();
-  VERIFY_ARE_EQUAL_STR(ValidNonDLLPath.c_str(), __FILE__);
+  std::string FilePath;
+  Unicode::WideToUTF8String(
+      hlsl_test::GetPathToHlslDataFile(L"lit.local.cfg").c_str(), &FilePath);
+  VERIFY_ARE_EQUAL_STR(ValidNonDLLPath.c_str(), FilePath.c_str());
   VERIFY_IS_TRUE(ExtSupportValidNonDLLPath.dxilDllFailedToLoad());
 
   // 4. Test production of class IDs CLSID_DxcCompiler, CLSID_DxcLinker,