Use Catch2 in libres tests

[pinkwah]

Replace test_area_context with off-the-shelf Catch2 testing.

Blockers:

• Install Catch2 (via conan)
• util_abort does abort or setjmp. Use C++ exceptions instead
• Some tests aren't run today. Remove / fix them.
• Add Catch2 test-data creation framework (for making ERT configs on the fly)

Progression:

• Analyse low-hanging, easy-to-convert-to-Catch2 tests
• If the test is using test-data files, move them to the C++ test code
• Start with each test separately. When done, comment on this discussion on your thoughts on how to improve test
• When most tests are catch2, we can look at improving the test-suite coverage (ie, rewrite the tests to do good things instead of evil things)

Relevant:

• Some Equinor tests are embarrassingly easy to make on-the-fly synthetised data
• Add gcov code coverage inspection for C tests.
• Ditto, for Python tests which hit the C code.
• Upload gcov to codecov.