Two issues are tackled:
1. The calculation of the correct length to print is fixed to treat the
precision as a maximum length instead as the exact length.
This is done for both qstr (%q) and for regular str (%s).
2. Fix the incorrect use of mp_printf("%.*s") to mp_print_strn().
Because of the fix of above issue, some testcases that would print
an embedded null-byte (^@ in test-output) would now fail.
The bug here is that "%s" was used to print null-bytes. Instead,
mp_print_strn is used to make sure all bytes are outputted and the
exact length is respected.
Test-cases are added for both %s and %q with a combination of precision
and padding specifiers.
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