libvpx_test: ensure rtcd init functions are called

libvpx_test: ensure rtcd init functions are called

In addition to allowing tests to use the RTCD-enabled functions (perhaps transitively)
without having run a full encode/decode test yet, this fixes a linking issue with
Apple's G++ whereby the Common symbols (the function pointers themselves) wouldn't
be resolved. Fixing this linking issue is the primary impetus for this patch, as none
of the tests exercise the RTCD functionality except through the main API.

Change-Id: I12aed91ca37a707e5309aa6cb9c38a649c06bc6a