test.bash: lock against multiple parallel incovations

Running multiple test.bash in parallel causes all kinds of
mayham.

test.bash

@@ -3,9 +3,22 @@
 set -eu
 
 cd "$(dirname "$0")"
+MYNAME=$(basename "$0")
+TESTDIR=/tmp/gocryptfs-test-parent
+LOCKFILE=$TESTDIR/$MYNAME.lock
 
-# Clean up dangling filesystem
-for i in $(cat /proc/mounts | grep /tmp/gocryptfs-test-parent | cut -f2 -d" "); do
+mkdir -p $TESTDIR
+
+(
+# Prevent multiple parallel test.bash instances as this causes
+# all kinds of mayham
+if ! flock --nonblock 200 ; then
+	echo "Could not acquire lock on $LOCKFILE - already running?"
+	exit 1
+fi
+
+# Clean up dangling filesystems
+for i in $(cat /proc/mounts | grep $TESTDIR | cut -f2 -d" "); do
 	echo "Warning: unmounting leftover filesystem: $i"
 	fusermount -u $i
 done
@@ -16,10 +29,12 @@ go test ./... $*
 
 # 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
+rm -Rf --one-file-system $TESTDIR
 
 if go tool | grep vet > /dev/null ; then
 	go tool vet -all -shadow .
 else
 	echo "\"go tool vet\" not available - skipping"
 fi
+
+) 200> $LOCKFILE