[mlir][test] Fix -Wunused-variable in PassBuilderCallbacksTest.cpp (NFC)

llvm-project/llvm/unittests/MIR/PassBuilderCallbacksTest.cpp:333:10:
error: unused variable 'Ret' [-Werror,-Wunused-variable]
    bool Ret = MIR->parseMachineFunctions(*Mod, MMI);
         ^
1 error generated.