Test cases for 'RegEx' are moved from 'test_string.h' to own test suite
in 'test_regex.h'. Additionally, new tests are introduced and '_init' is
removed as it isn't implemented/used anywhere.
`test_macros.h` is created to provide various macros acting as aliases for
doctest macros to work better with Godot internals and conventions. This also
makes it accessible for those who'd like to start writing tests in Godot, as
most vital information can be put together and documented directly in
`test_macros.h` header.
Developers are encouraged to include the new `tests/test_macros.h` for writing
new tests over `thirdparty/doctest/doctest.h`.
Added `TEST_CASE_PENDING("name")` as an alias for
`TEST_CASE("name", doctest::skip())` which could be used to mark failing tests
for issues yet to be fixed, so as to not affect CI testing results.
Added `ERR_PRINT_OFF` and `ERR_PRINT_ON` to control error printing for testing
various **expected** failure paths within Godot without polluting the test
summary with error messages.
