Check printf format strings in str_format_convert_test

Check printf format strings in str_format_convert_test

Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/9fed77a6fea29b8c8468bd41c6259c7f67163a65

Add ABSL_PRINTF_ATTRIBUTE to appropriate functions in
strings/internal/str_format/convert_test. Correct
TypedFormatConvertTest.Char, which was accidentally passing values of
types larger than int to StrPrint.

The author works at Google. Upstream applied this patch as Piper
revision 439388148 and exported it to GitHub; the Applied-Upstream URL
above points to the exported commit.

Gbp-Pq: Name str-format-convert-test-printf.diff