No TCK coverage for exception mappers when subresource interface methods are called

[cghislai]

I noticed that the resteasy implementation did not call a registered ResponseExceptionMapper when subresource interface methods are called; while it works fine for normal resources.

I suggest adding a test for this use-case; and maybe other providers could be tested on subresources as well.

See resteasy/resteasy-microprofile#239 (comment) for the resteasy issue.