tst/sys.t: fix 'tac -r -s' ERE: use '!+', not '!*'
tst/sys.t: fix 'tac -r -s' ERE: use '!+', not '!*'

Busybox awk behaves differently from other awks if given a
field separator ERE matching zero or more characters; it splits by
individual characters, instead of ignoring the null string match
like other awks do.

It is not at all clear to me that the busybox behaviour is a bug:
https://www.mail-archive.com/austin-group-l@opengroup.org/msg05571.html

So for these regression tests, change to a conceptually more
sensible ERE, '!+', matching one or more '!' characters. This fixes
three regression test failures when using Busybox awk for 'tac'.

modernish/modernish