test_draw_text: Fix typo introduced in 1dd83bc5

This wasn't actually leaving the `stbt run` process alive (presumably
because the `rm fifo` caused the process to exit with sigpipe?), but
it's worth fixing anyway.
tests: Exit status 77 indicates a skipped test

The number 77 is consistent with automake:
http://www.gnu.org/software/automake/manual/html_node/Scripts_002dbased-Testsuites.html
test_detect_match_visualisation: Use fifo instead of vp8enc/webmmux

This is faster, it works on OS X, and it also fixes a potential race
condition. See 1dd83bc5 for details.