Don't use a subset in test fetcher if on main branch

[ydshieh]

Feature request

Don't select a subset from the detected tests to run when they are many - if we are on the main branch.
This could detect any issue as early as at the merge time, not at the nightly run.

This is implemented in #28816 / #29673, but reverted in #29682 as we have

/bin/bash: line 25: /usr/bin/jq: Argument list too long
Exited with code exit status 126

as there are too many models and the arguments to pytest is too long.

We should find a way to overcome this in order to achieve our goal.

[ydshieh]

Fixed in #29710