mount: run tests in a subprocess to fix deadlock - fixes #3259

Before this change we ran the tests and the mount in the same process.
This could cause deadlocks and often did, and made the mount tests
very unreliable.

This fixes the problem by running the mount in a seperate process and
commanding it via a pipe over stdin/stdout.

vfs/vfstest/write_unix.go

@@ -46,7 +46,7 @@ func TestWriteFileDoubleClose(t *testing.T) {
 
 	// write to the other dup
 	_, err = unix.Write(fd2, buf)
-	if run.vfs.Opt.CacheMode < vfscommon.CacheModeWrites {
+	if run.vfsOpt.CacheMode < vfscommon.CacheModeWrites {
 		// produces an error if cache mode < writes
 		assert.Error(t, err, "input/output error")
 	} else {