ShellIT: always append exit to generated command files (#32)

[cansin]

Summary

Fixes #32.

Integration tests are defined by .in files in
use-gui/src/it/resources/testfiles/shell. Until now every .in file had to end
with an explicit exit, or the generated command file (and therefore the test
run) would not terminate. ShellIT now always appends an exit to the
generated command file, so test authors no longer need to add one.

Details

createCommandFile appends exit to every generated command file. USE echoes
the command, so the appended exit is added to the expected output, except:

• Invalid-model tests — the model fails to load and USE exits by itself
  before the command file is read. These are marked with a #expected exit
  comment and do not expect the (never executed) appended exit
  (t053, t073, t085, t088, t098, t104).
• Tests already ending in exit/quit/q — the test's own terminator runs
  first, so the appended exit is never executed (and not double-counted).

Because the appended exit terminates USE before its quiet-mode (-q)
end-of-input check, the valid-model tests that relied on that implicit check
now run check explicitly (t126–t132).

Testing

• New unit tests in ShellIT for createCommandFile: the appended exit is in
  both the command file and the expected output; #expected exit suppresses the
  expectation; exit/quit are not duplicated.
• The full ShellIT suite passes: 129 shell tests + 5 unit tests = 134.