selftests: prctl: Add new prctl test for PR_SET_NAME

This patch will add the new test, which covers the prctl call
PR_SET_NAME command. The test tries to give a name using the PR_SET_NAME
call and then confirm it that it changed correctly by using  PR_GET_NAME.
It also tries to rename it with empty name.In the test PR_GET_NAME is
tested by passing null pointer to it and check its behaviour.

Signed-off-by: Osama Muhammad <osmtendev@gmail.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>