[Clang] use unsigned integer constants in unit-test | fixes build error on...

[Clang] use unsigned integer constants in unit-test | fixes build error on ppc64le-lld-multistage-test

Fixes:
    /home/buildbots/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1526:11: warning: comparison of integer expressions of different signedness: ‘const unsigned int’ and ‘const int’ [-Wsign-compare]
    /home/buildbots/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1526:11: warning: comparison of integer expressions of different signedness: ‘const long unsigned int’ and ‘const int’ [-Wsign-compare]

Reviewed By: cor3ntin

Differential Revision: https://reviews.llvm.org/D156224