[libc] clean up sprintf macros and float tests (#110759)

The sprintf tests have a macro named "ASSERT_STREQ_LEN" which was used
in about half of the tests. This patch moves all of the tests which can
to using that macro. This patch also enables long double tests for %e
and %g, since those never got finished. There's still some work to do
enabling long double testing for long doubles other than the intel 80
bit format, but that can land in a followup.

The `#ifdef LIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION` lines are for a
followup patch.

Author: michaelrj-google