Based on /meta-selftest/recipes-test/emptytest/emptytest.bb, manipulate this recipe so that, for example, it breaks during compile phase.

See: https://wiki.yoctoproject.org/wiki/Oe-selftest#appending_to_a_recipe and 
meta/lib/oeqa/selftest/lic-checksum.py
for details on how to manipulate the recipe.

This can be used for bitbake/toaster/etc testing.

Example of test:
Basic test case that tries bitbaking the broken recipe and verifies that it fails in the expected way.

This can be expanded with test on broken patching, broken web location for the tarball, etc.

Another approach would be a fixed broken recipe that can be used for this kind of tests (one for each kind of test).