Migrate to doctest or other unit testing frameworks which supports C++11

[fktn-k]

Currently, the unit tests for fkYAML depends on Catch2 (v2).
No actual issues have arisen but depending on Catch2(v2) causes:

• long build time (I'm not sure if this is due to Catch2 though), and
• a worry about how long the v2 branch is available (since v3 doesn't support C++11)
  So, I would like to migrate to some unit testing framework which supports C++11 such as doctest which has also similar APIs as Catch2's.

[fktn-k]

It's turned out that doctest doesn't implement generators like GENERATE(...) on which the test suite for fkYAML heavily depends.
Other features seem to be sufficient for migration.