create tests that stress manifest merging

in #848 we're more aggressively leaning on info passing through the manifest and getting merged, but no tests actually check that manifest merging works right.

This is annoying to Properly test but that PR at least teaches --artifacts=lies to produce proper fake data, so we can perhaps write a test that basically runs:

* --artifacts=lies --target=abc
* --artifacts=lies --target=cde

And check that they combine right...? (Unclear if this will work exactly right, since lies is "like --artifacts=all" and tries to turn on everything...)

Or just straight up write some normal unit tests.

Start by reading PR #848 and the existing manifest-merging tests or unit-test structure. Try the --artifacts=lies runs with --target=abc and --target=cde, or use normal unit tests, and verify that the resulting manifest information combines correctly.

Written by the indexing model from the issue text.