tests: build error w/ higher gcc & lower gtest

tests: build error w/ higher gcc & lower gtest

If you have gcc >= 11 and gtest <= 1.8, you may have build errors:

/usr/src/gtest/src/gtest-death-test.cc: In function ‘bool testing::internal::StackGrowsDown()’:
/usr/src/gtest/src/gtest-death-test.cc:1009:24: error: ‘dummy’ may be used uninitialized [-Werror=maybe-uninitialized]
 1009 |   StackLowerThanAddress(&dummy, &result);
      |   ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
/usr/src/gtest/src/gtest-death-test.cc:999:6: note: by argument 1 of type ‘const void*’ to ‘void testing::internal::StackLowerThanAddress(const void*, bool*)’ declared here
  999 | void StackLowerThanAddress(const void* ptr, bool* result) {
      |      ^~~~~~~~~~~~~~~~~~~~~
/usr/src/gtest/src/gtest-death-test.cc:1007:7: note: ‘dummy’ declared here
 1007 |   int dummy;
      |       ^~~~~

As gtest's code is not our test target, let the compiler ignore warnings of gtest codes.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>