[SPIR-V] Ensure that Module resource is managed locally wrt. a unit test case and fix a memory leak (#123725)
Adding SPIRV to LLVM_ALL_TARGETS
(https://github.com/llvm/llvm-project/pull/119653) revealed a series of
minor compilation problems and sanitizer complaints. This PR is to move
unit tests resources (a Module ptr) from the class-scope to a local
scope of the class member function to be sure that before the test env
is teared down the ptr is released.