Extend current coverage for b/u tests more explicitly verifying
resulting type (`type(·) is ...` instead of `isinstance(·, ...)`),
verifying unicode(bstr)->ustr and bytes(ustr)->bstr, and str() of both
bstr and ustr.

Move the check for "no custom attributes" from test_qq to generic
test_strings_basic, because now verified string types are publicly
accessible, not only via qq.

Small cosmetics in benchmarks - by reusing hereby introduced xbytes()
utility.

No change for the code itself - the tests just add verification to
current status.