Add C API test support

This PR is similar to PR #859 by @carsonmcdonald.
I told "tests can implement in mrbgems." but then we can't test if DISTABLE_GEMS was set.

I changed the placeholder for C API tests.
So formar test directory will keep well-ordered.

This patch is invaluable to fix #1087.
I added test case for mrb_format() as an example.