validate: Remove the default testsuite implementation

validate: Remove the default testsuite implementation

The default testsuite implementation should belong to the default
asset repo where we have the corresponding knowledge.

We should style manage a sensible list of known blacklisted tests,
encoding profiles, and generators in GstValidate itself and allow testsuite
actual implementations to easily use them though the register_default_*
methods.

This allow us to be able to remove the ugly execfile() call.