Run Pytest with specific phase(s)/fork(s) tests

[hwwhww]

Feature

Add Pytest CLI option --fork so that we can just run with specific phase(s)/fork(s) tests.

p.s. not strongly, but I like --fork rather than --phase here and wish to remove terminology "phases" one day. But open to discuss if anyone likes --phase or other names more.

Examples

Example 1: by default, it runs all available phase(s)/fork(s):

pytest eth2spec/test/phase0/sanity/test_slots.py::test_slots_1

Example 2: only run merge tests:

pytest eth2spec/test/phase0/sanity/test_slots.py::test_slots_1 --fork=merge

Example 3: only run phase0 and altair tests:

pytest eth2spec/test/phase0/sanity/test_slots.py::test_slots_1 --fork=phase0 --fork=altair 

[djrtwo]

I concur on the fork vs phase. It is a bit confusing in the repo right now

[djrtwo]

I'm going to merge these PRs because they look good and I want my fast CI!