Previously test_strings_mod_and_format was testing % and .format via
compareing bstr and ustr results with similar result for unicode. 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
e.g. result of % will be compared to result of ustr % which opens the
door for bugs to stay unnoticed.

-> Adjust the test, similarly to 9a075b17 (golang_str: tests: Make
test_strings_methods more robust with upcoming unicode=ustr), 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 % and .format match it.