Do not fail build on newer MSVS on GTEST_SKIP macro

[SergeyRyabinin]

Issue #, if available:

...\aws-cpp-sdk-core-tests\utils\threading\ReaderWriterLockTest.cpp(94): error C2220: the following warning is treated as an erro
r [...\aws-sdk-cpp\build\tests\aws-cpp-sdk-core-tests\aws-cpp-sdk-core-tests.vcxproj]
...\aws-sdk-cpp\tests\aws-cpp-sdk-core-tests\utils\threading\ReaderWriterLockTest.cpp(94): warning C4702: unreachable code [...\aws-sdk
-cpp\build\tests\aws-cpp-sdk-core-tests\aws-cpp-sdk-core-tests.vcxproj]

Description of changes:
pragma ignore warning in AwsGTestSuite header (should be used only in tests).
Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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