tests: fix arm regression due to kernel not found

tests: fix arm regression due to kernel not found

At Ia49298304f658701ea0800bd79e08db404a655c3 we removed the default
kernel and DTB filenames from FSConfig.py.

However, the regression tests rely on that to find those blobs.

This commit restores those default filenames just for the config of the
regression tests.

Change-Id: I9d7d869b0087ee8a3b63088693f753a703ead5d6
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15957
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>