test_that_press_times_out...: Improve error message
test_that_press_times_out...: Improve error message

I saw this test fail several times, not because `stbt.press()` didn't
time out --that's what we *want* to happen-- but because `stbt.press()`
didn't time out yet and so the entire `stbt run` process was killed by
`timeout`.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester