GTestConvertor using legacy version of Google Test

[mickeykkim]

References of TestCase should be changed to TestSuite in GTestConvertor.h. TestCase is Google Test legacy API and should be updated to reflect the current version of Google Test.

Otherwise this error occurs:

shared/libs/googletest/googletest/include/gtest/gtest.h:199:7: error: typedef redefinition with different types ('testing::TestSuite' vs 'testing::TestCase')
using TestCase = TestSuite;
      ^
cpputest/include/CppUTestExt/GTestConvertor.h:57:11: note: previous definition is here
    class TestCase;

#1470 was created to introduce this change.