tests: Disable blkid cache usage

Using the blkid cache can cause issues when running many file system tests
in parallel. We do not need it, as its only there to improve performance,
and using the cache does not provide significant performance improvements.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

tests/util/grub-fs-tester.in

@@ -1,6 +1,7 @@
 #!@BUILD_SHEBANG@
 
 set -e
+export BLKID_FILE=/dev/null
 
 fs="$1"