`std::is_pod<T>` was deprecated in C++20
original (pre `is_pod`) error on GCC 4.8:
```
/fmt/test/gtest/gtest.h: In static member function 'static constexpr bool testing::internal::MatcherBase<T>::IsInlined()':
/fmt/test/gtest/gtest.h:6512:12: error: 'is_trivially_copy_constructible' was not declared in this scope
std::is_trivially_copy_constructible<M>::value &&
^
/fmt/test/gtest/gtest.h:6512:45: error: expected primary-expression before '>' token
std::is_trivially_copy_constructible<M>::value &&
^
/fmt/test/gtest/gtest.h:6512:46: error: '::value' has not been declared
std::is_trivially_copy_constructible<M>::value &&
^
```
* all GTest/GMock files moved to `test/gtest` directory
* `CMakeLists.txt` created in `test/gtest` from `CMakeLists.txt` in `test`
* GTest/GMock target in CMake renamed to `gtest` (was `gmock`)
* CMake `gtest` target updated to export includes as "gtest/gtest.h" or "gmock/gmock.h" only
* includes in tests updated: "gtest.h" -> "gtest/gtest.h", "gmock.h" -> "gmock/gmock.h"
* removed duplications of `target_include_directories` for GTest/GMock directories (CMake manages them)
