lib: tests: Add test for spinlocks

Implement the test which covers some of the functions from the
`riscv_locks.h` file. This test consists of 3 test cases:

1) For lock/unlock functions
2) Unsuccessful trylock (the lock was previously taken)
3) Successful trylock (the lock is free and can be taken)

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>