Add libcnb-cargo and libcnb-test tests for cross-compile assistence help text

[edmorley]

Recently (in #704) there was a regression in error handling for libcnb-test which affected various failure modes including those seen when cross-compile tools aren't installed.

This has since been fixed in #720 and some more integration tests for general packaging error handling were added in #717.

However, there is one last scenario that's not yet integration tested: the assistance help message shown when cross compile tools aren't installed.

Testing this in CI isn't as simple as the other scenarios, since for it to be integration tested, we need the tools to not be installed (as it's not really practical to use other tricks like overriding PATH due to the parallel nature of cargo test and changing env vars affecting the whole test process).

As such, we'll likely need a separate CI job, or to run a subset of the tests prior to the apt-get install musl-tools CI setup step. We'll then also need to make sure that (a) the test of the cross-compile message doesn't fail when the tools are actually installed (where it's expected that there's no help text), plus (b) that the tests can't accidentally not be run (due to how cargo test <name of test> silently passes if the test gets renamed in the meantime etc.

GUS-W-14438781.