rename test files

--HG--
rename : tests/roots/test-only-directive/conf.py => tests/roots/test-directive-only/conf.py
rename : tests/roots/test-only-directive/contents.rst => tests/roots/test-directive-only/contents.rst
rename : tests/roots/test-only-directive/only.rst => tests/roots/test-directive-only/only.rst
rename : tests/test_only_directive.py => tests/test_directive_only.py
rename : tests/test_cpp_domain.py => tests/test_domain_cpp.py
rename : tests/test_std_domain.py => tests/test_domain_std.py