[test] add copyright year test on TEST_SLOW

for GH #719. most files really have wrong years
[test] speedup copyright year test

use only --format="%ai", use -n1 for the non-reverse case

The results are at https://github.com/parrot/parrot/issues/719
=> 1478 wrong copyright years
[test] add documentation to TEST_SLOW
[test] re-enable t/pmc/signal.t
[test] Re-enabled instable signals tests as TODO

signals do work fine manually, just the tests not.
eg. on darwin:
$ (./parrot t/pmc/signal_1.pasm &); ps | grep "./[p]arrot" | cut -c1-6 | xargs kill -HUP
start
$ (./parrot t/pmc/signal_3.pasm &); ps | grep "./[p]arrot" | cut -c1-6 | xargs kill -HUP
start