fix #284: make gtest an external dependency AND upgrade gtest 1.10.0 -> 1.12.1

[defaultbranch]

Regarding issue #284 (and maybe also older issues like #233):

• this PR drops the GoogleTest code copied into the Entwine code
• and replaces it with CMake code according to https://google.github.io/googletest/quickstart-cmake.html#set-up-a-project

Please consider this pull request a suggestion and try for yourself whether it works; Caution:

• this deletes a whole folder from the project (I believe this was copy-paste content)
• this also upgrades from GoogleTest 1.10.0 to 1.12.x
• and some test is failing (for me) and may need additional fixing (which I cannot provide, since I am not an Entwine developer, only a user)

[connormanning]

Might just need to update the JSON library used, because it looks like the error is not in Entwine proper but rather this issue. As to why it is only appearing now and not before, I do not know.

[defaultbranch]

Ah sorry, I was working in parallel and just saw your reply now, but came to sth similar (#286; just close if redundant)