unicode: make utf8 test count static

unicode: make utf8 test count static

The variables failed_tests and total_tests are not used outside of the
utf8-selftest.c file so make them static to avoid the following warnings:

fs/unicode/utf8-selftest.c:17:14: warning: symbol 'failed_tests' was not declared. Should it be static?
fs/unicode/utf8-selftest.c:18:14: warning: symbol 'total_tests' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20240308183215.1924331-1-ben.dooks@codethink.co.uk
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>