An eyepiece that you can rotate into the return beam to examine the focused image is very helpful! I use an old 25mm Kellner. Frost the light blub well (fine sandpaper seems to work); otherwise the illumination across the slit (and the mirror under test) won't be even.
The advantage of a slit tester seems to me to be not only a brighter mirror image to test, but also that you can spot a turned down edge easily (Texereau talks about this).
Roger Ceragioli@uh.edu