test-check-sr-results: robustify before CI

If the systemready-scripts repository is cloned before installing
dt-validate (which is the case with gitlab CI), the .dtb.log is re-made,
which breaks the unit test.
Fix this by forcibly touch'ing the .dtb.log in the unit test.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>