fix: test: fix out-of-tree mem_test

Previously changed mem_test (!10320) introduces a test which checks for
the value of `__FILE__`, which is different if the build is done
out-of-tree or not, even though this is not relevant for the test (only
the base filename is). This result in a broken test for out-of-tree
builds. Fix this by changing the way the "grep" is done in the test,
ignoring the optional path prefix in the filename.

Merge branch 'colin-fix-outoftree-memtest' into 'main'

See merge request isc-projects/bind9!10343