selftests: allow runners to override the timeout

The default timeout for selftests tests is 45 seconds. Although
we already have 13 settings for tests of about 96 sefltests which
use a timeout greater than this, we want to try to avoid encouraging
more tests to forcing a higher test timeout as selftests strives to
run all tests quickly. Selftests also uses the timeout as a non-fatal
error. Only tests runners which have control over a system would know
if to treat a timeout as fatal or not.

To help with all this:

  o Enhance documentation to avoid future increases of insane timeouts
  o Add the option to allow overriding the default timeout with test
    runners with a command line option

Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
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>