Add the foundation for automated unit testing using the Google Test framework

[Kardbord]

Lots of changes in this one due to a pretty big reorganization, as well as the fact that my new neovim setup feels very strongly about the way things should be formatted. 😅 Still working out the kinks there.

Primarily, the purpose of these updates is to get us set up to write unit tests for the parser so we don't have to eyeball the output of spicy-driver manually each time we make a change. We can also set up a GitHub action to run the tests automatically on each PR and after each merge.

Changes:

• Consolidated gtest, test data, and btest all under the testing directory
• Added CMake logic to
  • generate C++ headers/sources from spicy, and link our tests against them
  • generate C++ headers containing embedded binary data from our .binpb test data (I may have gotten carried away here, but it beats hard coding relative paths into the unit tests)
• Added a bare-bones unit test

Leaving this in a draft state for now, as it is blocked by zeek/spicy#1752.

[Kardbord]

Still need to add unit tests for i32 and i64 wire types.

[TnekRex]

Pulled down code base. Verified unit tests compile, run and all tests pass. I'll take a look at the implementation details as time permits - maybe take a shot at implementing the unit tests for I32 and I64 wire types as an exercise to get my head around gtest.