file_test: make also "linux" a valid match

The output of file for vmlinu* varies based on architecture
and presumably also its own version. Make this testcase
more tolarant by matching also "linux". This fixes testcase
on s390 for at least file-5.11 up to file-5.19.

Signed-off-by: Jan Stancek <jstancek@redhat.com>