sparsify: Fix test-virt-sparsify-in-place-fstrim-unsupported.sh

Since Linux commit f663b5b38fff trimming vfat is now supported by
Linux.  This broke the test which assumed it was not supported.  Use
another filesystem (minix) which does not support trimming instead.

Thanks: Daniel P. Berrangé and Pino Toscano.

tests/test-functions.sh

@@ -198,6 +198,16 @@ skip_unless_feature_available ()
     fi
 }
 
+# Skip if a filesystem is unavailable in the daemon.
+skip_unless_filesystem_available ()
+{
+    r="$(guestfish -a /dev/null run : filesystem_available "$1")"
+    if [ "$r" != "true" ] ; then
+        echo "$(basename $0): test skipped because $1 filesystem is not available"
+        exit 77
+    fi
+}
+
 # Skip unless the libvirt minimum version is met.
 skip_unless_libvirt_minimum_version ()
 {