Add test which checks megadep::megadep correctly reads and processes the
includes from a Rattlehead-generated project.

Change Log:
* Add explicit write_direct_includes and write_indirect_includes methods
  for MegadepPolicy in megadep::megadep.
* Add rm_rf function in tests/src/utilities/rm_rf.hpp, which implements
  rm -rf. Replace manual file management in tests with rm_rf.
* Move some console output from megadep::megadep to megadep.main.cpp.
* Add megadep::librattlehead alias library

The Rattlehead project is intended to be used to test the correctness of
Megadep, and to benchmark it in a fine-tunable manner.

Rattlehead generates test project layouts with nested directories and
acyclic dependencies between files.
Project files contain #include directives to a number of other files in the
project to create dependencies between files, suitable for consumption by
Megadep.

Rattlehead has been added in tests/src.

Change Log:
* Add .gitlab-ci.yml and CI scripts under tests/ci.
* Add CI tests for lint.sh, format.sh, and unit tests
* Fix missing headers
* Drop CMake version to 3.13 for Ubuntu 19.04

git-subtree-dir: external/cli11
git-subtree-split: 13becaddb657eacd090537719a669d66d393b8b2

filesystem.cpp: Use reused path buffer for directory recursion. Use std::string_view for input directory name type, and std::string for returned filename type. Remove common_types.hpp.

git-subtree-dir: external/catch2
git-subtree-split: fbf03cd31c6a1c211fa85bd620defbdc4c2e37d0