Split build production vs testing

[neatudarius]

Split build production vs testing: #28

• Remove gtest submodule. Use Cmake FetchContent_Declare instead (gtest tag = $latest official release https://github.com/google/googletest/releases/tag/v1.14.0).
• Use cmake BUILD_TESTING (default ON - build and run tests; can be set to OFF).
• Remove redundant CMakeLists.txt as discussion in discourse
• Add build docs.

[steve-downey]

With optional being a template, I'm concerned that not building tests will even more easily lead to silent breaks in building the library that only show up in users of the library. Although optional is pretty simple, I wouldn't want to encourage packagers to skip tests, even by accident. It's more important that a Conan or Vcpkg build self-test because it's even more likely that something is outside of our test matrix.

I do have some code from an earlier version of my scratch project that will check if the gtest project exists before enabling the code, though.

Might also switch, temporarily, to pulling googletest in as a git subtree rather than as a git submodule, which would mean we have a copy, but can also pull in updates, at least while we sort out a package reuse mechanism. Or switch to something lighter weight to vendor in.

[neatudarius]

With optional being a template, I'm concerned that not building tests will even more easily lead to silent breaks in building the library that only show up in users of the library. Although optional is pretty simple, I wouldn't want to encourage packagers to skip tests, even by accident. It's more important that a Conan or Vcpkg build self-test because it's even more likely that something is outside of our test matrix.

I'm OK with both approaches, but I would like to use an uniform approach for all libraries in Beman - CC: @camio , maybe we can add this topic to our sync agenda. Also, I propose to move this discussion in https://discourse.bemanproject.org/t/build-tests-always-implied-or-optional-task/144.

[camio]

I mentioned a couple minor things, but it otherwise looks good. Note that I didn't review the docker file at all.

[neatudarius]

I mentioned a couple minor things, but it otherwise looks good. Note that I didn't review the docker file at all.

Sorry, I added the Dockerfile by mistake. I don't want to added right now. Currently I'm using it to replicate the environment from CI, but it's not ready. TBD