The `debug` build target (`MRB_GC_STRESS` is enabled) on CI have been
compiled but not tested so far. However, I think testing with
`MRB_GC_STRESS` is effective (in fact, I found #4907 bug).

Therefore, I integrated `debug` and `full-debug` build targets to enable
`MRB_GC_STRESS` testing. Testing with `MRB_GC_STRESS` takes a little time,
but compiling takes more time, so CI execution time does not increase due to
decrease of build target.