tests: lib: c_lib: add coverage for qsort_r

A user previously reported that `qsort_r()` did not have test
coverage. Prior to 845a200c
`qsort()` was actually just calling `qsort_r()` inline.

There is still virtually no difference between the two
sorting routines, but it would be good to add coverage.

Relates-to #44218

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>