Tests: Add case covering PCH in a unity build

Issue: #25650

Tests/RunCMake/PrecompileHeaders/PchInterfaceUnity-check.cmake

@@ -0,0 +1 @@
+include(${CMAKE_CURRENT_LIST_DIR}/PchInterface-check.cmake)

Tests/RunCMake/PrecompileHeaders/PchInterfaceUnity.cmake

@@ -0,0 +1,2 @@
+set(CMAKE_UNITY_BUILD 1)
+include(PchInterface.cmake)

Tests/RunCMake/PrecompileHeaders/RunCMakeTest.cmake

@@ -12,6 +12,7 @@ endfunction()
 run_cmake(DisabledPch)
 run_cmake(PchDebugGenex)
 run_test(PchInterface)
+run_test(PchInterfaceUnity)
 run_cmake(PchPrologueEpilogue)
 run_test(SkipPrecompileHeaders)
 run_test(CXXnotC)