Split build production vs testing

[neatudarius]

Split build production vs testing: #28

• if CMAKE_BUILD_TESTS is not defined, only the production code is built.
• if CMAKE_BUILD_TESTS is defined, pull googletest and build production code + tests.

[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.