fix clang tidy modernize-use-equals-default warnings

[jasjuang]

Prior to the fix using TEST_F will cause clang tidy warnings like

/home/jasjuang/sample/unit-test/SampleTest.cpp:23:1: warning: use '= default' to define a trivial default constructor [modernize-use-equals-default]
TEST_F(SampleTest, OnePlusOne)
^
/usr/include/gtest/gtest.h:2369:3: note: expanded from macro 'TEST_F'
  GTEST_TEST_(test_fixture, test_name, test_fixture, \
  ^
/usr/include/gtest/internal/gtest-internal.h:1358:58: note: expanded from macro 'GTEST_TEST_'
    GTEST_TEST_CLASS_NAME_(test_suite_name, test_name)() {}                   \
                                                         ^

This PR eliminates this warning.