fs: rename TestDirectIO to TestFUSEDirectIO

There are two types of DIRECT_IO. The one documented in open(2) and
FUSE's FOPEN_DIRECT_IO feature. This test exercises the latter.

Change-Id: Ie834b126318dedf5f65f0cf9805019ccda531797

fs/directio_test.go

@@ -52,7 +52,8 @@ func (f *dioFile) Open(ctx context.Context, flags uint32) (fh FileHandle, fuseFl
 	return &dioFH{}, fuse.FOPEN_DIRECT_IO, OK
 }
 
-func TestDirectIO(t *testing.T) {
+// this tests FOPEN_DIRECT_IO (as opposed to O_DIRECTIO)
+func TestFUSEDirectIO(t *testing.T) {
 	root := &dioRoot{}
 	mntDir, server, clean := testMount(t, root, nil)
 	defer clean()