As part of tier0 unification across the rhel-pt-gcc components
(RHEL-113756), introduce tier0 testing. The testing is limited
to x86_64-only (OSCI-imposed).
The tests selected for it are the public part of tier1. In other
words, we are introducing tier0 as the public part of tier1 run
on x86_64-only.
For awareness, RHEL builds get full tier1 in a later phase of gating
(BaseOS CI). This commit doesn't affect that in any way; we are
adding a small bit of functional testing sooner in the pipeline.
