tests: Warnings raised by clang++

[Jim Huang]

While I was using clang++ to build the builtin test suite, I encountered the following warnings:

tests/impl.cpp:9411:5: warning: unused typedef 'UBYTE_test_cases' [-Wunused-local-typedef]
    GENERATE_MM_CMPESTRI_TEST_CASES
    ^
tests/impl.cpp:9372:5: note: expanded from macro 'GENERATE_MM_CMPESTRI_TEST_CASES'
    ENUM_MM_CMPESTRI_TEST_CASES(UBYTE, ubyte, uint8_t)  \
    ^
tests/impl.cpp:9382:7: note: expanded from macro 'ENUM_MM_CMPESTRI_TEST_CASES'
    } type##_test_cases;                                                    \
      ^
<scratch space>:105:1: note: expanded from here
UBYTE_test_cases
^

Reproducible via make CXX=clang++ on both GNU/Linux and macOS.