selftests: static_keys: fix test name in messages

As a general rule, the name of the selftest is printed at the beginning
of every message.

Use "static_keys" (name of the test itself) consistently instead of mixing
"static_key" and "static_keys" at the beginning of the messages in the
test_static_keys script.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>