tests: use check_quota for invalid quotactl commands

tests: use check_quota for invalid quotactl commands

Use check_quota function for invalid commands instead of manual quotactl
syscall invocations. In this way successful injection can also be checked
for these commands in the next commit.

* tests/quotactl.c (main): Add invalid_cmd_str and invalid_id_str local
variables, remove unused variable rc, use check_quota instead of manual
quotactl syscall invocations.
* tests/quotaclt.c (main): Add invalid_cmd_str variable, remove unused
variable rc, use check_quota instead of manual quotactl syscall invocations.