overwrite CMake metadata for libabseil-tests

[h-vetinari]

Follow-up from #58 because it's not yet usable in conda-forge/libprotobuf-feedstock#144

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

So, I'm not a fan of clobbering at all, but I don't see how the alternatives would be better:

1. make libabseil-tests an independent output, not based on libabseil
   • would mean either being unable to co-install (hard blocker for using libabseil-tests where another dependency in the environment already run-depends on libabseil) or clobbering all artefacts when both end up being co-installed (way more risky IMO than clobbering just the metadata)
2. package the tests in libabseil proper
   • would mean we bring a lot of unnecessary stuff along everywhere, and also bump to MacOS 10.13 minimum
3. make libabseil contain the CMake metadata (and only that) for the test targets
   • errors would be confusing if your build depends on a test-target (like protobuf), but you only have libabseil installed, because the CMake metadata would be wrong, so we'd get some variation of linker errors or files not found.
   • might be an elegant solution, but this would need some very convoluted build scripts. It would be a big pain IMO to dynamically come up with the right list of files we'd need to delete for libabseil, after initially building everything with tests enabled.

CC @xhochy @isuruf @hmaarrfk

[h-vetinari]

I think the clobbering introduced here would be the least bad trade-off, especially as it's only going to happen for a really niche build dependency (no-one should ever be run-depending on the test targets).

[h-vetinari]

Actually, given that this is for a completely new output that's extremely niche, and a change with a very small blast radius, I'm going to merge this to be able to work on protobuf. I'll open up a follow-up issue to discuss this further, happy to implement any decision we come to afterwards.