golang_str: tests: Make test_strings_methods more robust with upcoming unicode=ustr

Previously test_strings_methods was testing a method via comparing bstr
and ustr results of .method() with similar result of unicode.method().
This works reasonably ok. However under gpython, when unicode will be
replaced with ustr, it will no longer compare results of bstr/ustr
methods with something good and external - indeed in that case bstr/ustr
.method() will be compared to result of ustr.method() which opens the
door for bugs to stay unnoticed.

-> Adjust the test to explicitly provide expected result for all entries
in the test vector. We make sure those results are good and match std
python because we also assert that unicode.method() matches it.