cmd/unity: provide more useful default commands

[myitcv]

Currently, unity testscript tests can use (without resorting to exec) the following commands:

• cue will run the version of cmd/cue under test
• the standard testscript commands like cd, chmod, cmp, ...

Some/many projects might well benefit from more advanced comparison tools like semantic diff, rather than the plain cmp.

This issue is an umbrella issue for adding any such "power" commands to the default environment for testscript tests.