add some "no invalidations" tests for the sysimage

[nsajko]

Test that defining certain handpicked type-method pairs (as a user) doesn't result in any invalidation of the sysimage.

Some of the tests are marked as broken.

Merging this will make Julia's contribution process more demanding: when someone makes a PR they might need to fix unrelated type stability issues, or decrease max_methods for some function, before being able to merge their initial PR. However that's better than ignoring the constant danger of type instability/invalidation regressions making the sysimage and precompilation less useful.

NB: this PR should only be merged while up-to-date with the Git master branch.