Add CEEDLING_IGNORE_SANITY_CHECK, which if defined skips test sanity check

This is required if using ceedling to run Unity's test suite, as in this
case the test counts don't match up (Unity's tests are allows to fail,
and this can be treated as a success, for example).
Merge pull request #32 from canton7/feature/ignore-sanity-check

Add CEEDLING_IGNORE_SANITY_CHECK, which if defined skips test sanity check