tests: libc: Update libc testcases to solve coverity issues

1. Refine some testcases for testing some libc functions, include
strlen(),strcmp(),strncmp(),memcmp(),memstr(),memcpy(),memmove()
strtol(),strtoul().
2. Change some code to make the code style consistent.

Fixes zephyrproject-rtos#33794
Fixes zephyrproject-rtos#33810
Fixes zephyrproject-rtos#33806

Signed-off-by: Ying ming <mingx.ying@intel.com>