tests: unmount leftover filesystems before starting the tests

A panic during the tests can leave mounted filesystems behind.

test.bash

@@ -4,13 +4,18 @@ set -eu
 
 cd "$(dirname "$0")"
 
+# Clean up dangling filesystem
+for i in $(cat /proc/mounts | grep /tmp/gocryptfs-test-parent | cut -f2 -d" "); do
+	echo "Warning: unmounting leftover filesystem: $i"
+	fusermount -u $i
+done
+
 source build.bash
 
 go test ./... $*
 
-# Clean up after ourself, but don't descend into possibly still mounted
-# example filesystems.
-# The tests cannot to this themselves as they are run in parallel
+# The tests cannot to this themselves as they are run in parallel.
+# Don't descend into possibly still mounted example filesystems.
 rm -Rf --one-file-system /tmp/gocryptfs-test-parent
 
 if go tool | grep vet > /dev/null ; then