proposal: testscript: allow to specify wanted exit status

[marco-m]

Hello,

today testscript allows to require:

• success (exit status 0) with exec foo
• failure (exit status != 0) with ! exec foo.

Sometimes one would like to specify the exact exit status value. For example, if curl was written in Go, see section "EXIT CODES" of the curl man page.

I realized that a backward-compatible and clear way to express this would be to add an optional integer to the right of the ! symbol:

• ! exec foo as of today
• !<N> exec foo, where <N> is a non-zero, positive integer, to specify the exit status.
  • For example: !2 exec foo requires foo to exit with status 2.

As usual, the proposed syntax is just a possibility, I care more about the feature itself.

Thanks for testscript!