Tests: Unset the PATH environment variable more explicitly where needed

Previously we relied on setting an environment variable to empty to
unset it on Windows.

Tests/RunCMake/CMP0194/CMP0194-common.cmake

@@ -1,3 +1,3 @@
 enable_language(C)
-set(ENV{PATH} "")
+unset(ENV{PATH})
 enable_language(ASM)

Tests/RunCMake/FindLua/FindLuaTest.cmake

@@ -9,8 +9,8 @@ set(ENV{CMAKE_INCLUDE_PATH} )
 set(ENV{CMAKE_PREFIX_PATH} )
 set(ENV{CMAKE_FRAMEWORK_PATH} )
 
-set(ENV{PATH} )
-set(ENV{INCLUDE} )
+unset(ENV{PATH})
+unset(ENV{INCLUDE})
 
 set(CMAKE_SYSTEM_INCLUDE_PATH )
 set(CMAKE_SYSTEM_PREFIX_PATH )