Add specs for signal exit

[straight-shoota]

We were missing generalized specs for Process::Status behaviour on a signal exit, particularly for #exit_code.
Predicate methods like #normal_exit? already have platform-specific tests, but this change enhances the portable portion of the spec for basic behaviour.

The behaviour of #exit_code might change in the future (see #15228), but this encodes the current status.

[straight-shoota]

It seems normal_exit? and signal_exit? are entirely broken on Windows.
Darn, Windows doesn't have signals of course. So normal_exit? is true for every status, but it doesn't necessarily corresponds to ExitReason::Normal 🤦

  1) Process::Status #normal_exit?
     Failure/Error: status_for(:interrupted).normal_exit?.should be_false

       Expected: false
            got: true

     # spec\std\process\status_spec.cr:50

  2) Process::Status #exit_code
     Failure/Error: status_for(:interrupted).exit_code.should eq 0

       Expected: 0
            got: -1073741510

     # spec\std\process\status_spec.cr:30

  3) Process::Status #signal_exit?
     Failure/Error: status_for(:interrupted).signal_exit?.should be_true

       Expected: true
            got: false

     # spec\std\process\status_spec.cr:60