test_config: Use Catch2's REQUIRE_THROWS_AS instead of manually dealing with it.
test_config: Use Catch2's REQUIRE_THROWS_AS instead of manually dealing with it.

Added tests to verify default option passthrough and getString() method.

curieos/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,lordofhyphens/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,curieos/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r