tests: Improve error message from `timeout`

The error message used to be:

    tests/run-tests.sh: line 52: 85551 Alarm clock: 14 perl -e 'alarm shift @ARGV;
         exec @ARGV;
         print "timeout: command not found: @ARGV\n";
         exit 1;' "$@"

Now it is:

    tests/run-tests.sh: line 52: 84877 Alarm clock: 14 "$testdir"/timeout.pl "$@"

Still not ideal (it doesn't show the command line that timed out), but
better.
test_runner_continues_after_uninteresting_failure: Increase timeout

This was timing out before `test.py` had been run 3 times (the third
time raises a `UITestFailure` so `run` should exit) when `make check`
was running several tests in parallel.
test_signalname: New selftest for `extra/runner`

Tests the `signalname` function from commit 6b99852f.