Accept ! for negation without requiring a space

[ldemailly]

Fixes #190

[mvdan]

Sorry, but I don't think this is a good idea. The testscript syntax is simple and it should stay that way. Neither POSIX Shell nor Bash even support removing the space after !, so this doesn't bring us any closer to syntax that people are used to. I'll review Axel's PR instead.

[ldemailly]

for the record, re consistency:

lines like

[!exec:false] skip

aren't

[ ! exec:false ] skip