fix: Make test executables optional

[joeyparrish]

When including zlib as a submodule, it is useful to be able to disable the tests.

[EmosewaMC]

It'd be great to be able to disable the tests when using this project as a submodule. I'm spending more time than I would like attempting to get these tests found by ctest so I can get back to working on other stuff. Just being able to disable them since they do not pertain to our project would be great.

[joeyparrish]

@madler, would you please look at this when you have time? We are currently forced to use a fork of zlib because of CMake issues like this.

[nmoinvaz]

Can you just do add_subdirectory(zlib EXCLUDE_FROM_ALL)?

[joeyparrish]

We are doing that: https://github.com/shaka-project/shaka-packager/blob/0ee4af438d4ff943cf747f083cd149223d2bd4c5/packager/third_party/CMakeLists.txt#L39

[nmoinvaz]

Actually, I just ran across this in another repository. Here is the associated Kitware issue: https://gitlab.kitware.com/cmake/cmake/-/issues/20212.

Alternatively, you could use zlib-ng which has options to disable testing, but it doesn't fix the problem for this repo.

[joeyparrish]

Yes, thank you for bringing this up. My PR description was poor, and should have had those details.

Here is what happens with EXCLUDE_FROM_ALL, but without this PR:

    Start 5: example
Could not find executable example
Looked in the following places:
example
example
Release/example
Release/example
Debug/example
Debug/example
MinSizeRel/example
MinSizeRel/example
RelWithDebInfo/example
RelWithDebInfo/example
Deployment/example
Deployment/example
Development/example
Development/example
Unable to find executable: example
5/6 Test #5: example ..........................***Not Run   0.00 sec
    Start 6: example64
Could not find executable example64
Looked in the following places:
example64
example64
Release/example64
Release/example64
Debug/example64
Debug/example64
MinSizeRel/example64
MinSizeRel/example64
RelWithDebInfo/example64
RelWithDebInfo/example64
Deployment/example64
Deployment/example64
Development/example64
Development/example64
Unable to find executable: example64
6/6 Test #6: example64 ........................***Not Run   0.00 sec

67% tests passed, 2 tests failed out of 6

Total Test time (real) =  23.97 sec

The following tests FAILED:
          5 - example (Not Run)
          6 - example64 (Not Run)

[joeyparrish]

I updated the PR description with more detail on why this is necessary. Thanks, @nmoinvaz, for the link to the cmake issue tracker!

[joeyparrish]

The change is now backward compatible for the sake of those who depend on the example and minigzip binaries. Only the add_test() calls have been made conditional on BUILD_TESTING. This has also been rebased and retargeted to the develop branch.

Thanks!