clang-tidy: issues in ZenohUTransportTest.cpp

[debruce]

1. do not use namespace using-directives; use using-declarations instead (line 23)
2. destructor should be annotated with override at line 40
3. unused variables: authority (line48)
4. TestBody has "cognitive complexity of 28 (threshold 25)". clang-tidy has failed the cognitive complexity test due to gtest macros, probably. This test case is as understandable as any.
5. Another bogus if-then-else complaint, but this time buried in gtest macros at line 84

clang-tidy was 95% useless on this gtest file.