gst-qa-override-registry: load overrides dynamically

gst-qa-override-registry: load overrides dynamically

Shared objects listed in GST_QA_OVERRIDE are loaded on startup,
and the symbol gst_qa_create_overrides is run. It should create
any override needed. While it can do anything it wants, this
is discouraged.

GST_QA_OVERRIDE should be a comma separated list of shared objects,
any relative paths should be from the current working directory
at the time they are loaded (ie, if the process to be traced
changes cwd, use absolute paths).

No attempt whatsoever is made at not running what was not meant.

Includes a sample shared object for illustration purposes.