libstdc++: Fix std::format test for Solaris [PR113450]

libstdc++: Fix std::format test for Solaris [PR113450]

When int8_t is a typedef for char (rather than signed char) this test
fails because it tries to format a char, which is treated differently
from formatting other integral types (including signed char).

Use signed char explicitly so the result doesn't depend on the
non-portable definition of int8_t.

libstdc++-v3/ChangeLog:

	PR libstdc++/113450
	* testsuite/std/format/functions/format.cc: Use signed char
	instead of int8_t.