I tried calling this script with "tests/foo.lua" as an argument, but it actually
expects "foo.lua" since it changes into the appropriate directory itself.
Because this file does not exist, "cat $f | awesome-client" failed and thus this
whole script just hung (the test runner was never actually started).
Fix this by testing for the test-file to exist before trying to run it.
Signed-off-by: Uli Schlachter <psychon@znc.in>