cmake: Fix missing gtest linkage, missing functions.

cmake: Fix missing gtest linkage, missing functions.

During linking the error is:
```
/usr/bin/ld.lld: error: undefined symbol: testing::Message::Message()
>>> referenced by test_compression.cc:78 (/home/jenkins/workspace/ceph-luminous/src/test/compressor/test_compression.cc:78)
>>>               CMakeFiles/unittest_compression.dir/test_compression.cc.o:(CompressorTest_small_round_trip_Test::TestBody())

/usr/bin/ld.lld: error: undefined symbol: testing::internal::AssertHelper::AssertHelper(testing::TestPartResult::Type, char const*, int, char const*)
```
This can probably be fixed by backporting
    https://github.com/ceph/ceph/pull/23628
and friendsr. But that needs a lot of fixing.

So instead of backporting, is this PR a lot easier

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>