selftests: firmware: add empty string and async tests

Now that we've added a 'trigger_async_request' knob to test the
request_firmware_nowait() API, let's use it. Also add tests for the
empty ("") string, since there have been a couple errors in that
handling already.

Since we now have real ways that the sysfs write might fail, let's add
the appropriate check on the 'echo' lines too.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>