tests: fix grep for checks error messages

I noticed the error type passed in didn't matter for check tests to pass.
There's a couple of problems with the grep regex. The error/warning
messages begin with the output filename now, so "ERROR" or "Warning" is not
at the beginning of the line. Secondly, the parentheses seem to be wrong.
It's not clear to me what was intended.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>