--fork cli option: fix the case of using pytest -k without setting directory path

[hwwhww]

Issue

1. --fork is action="append" option without a single default value.
   • request.config.getoption("--fork") returns None if directory path is set.
   • request.config.getoption("--fork") raises ValueError if directory path is unset. (Some AttributeError in pytest internally)
2. If anyone wants to run with -k AND without setting file or path, it raises error:

pytest -k test_slots_1

request = <SubRequest 'run_phases' for <Function test_slots_1>>

    @fixture(autouse=True)
    def run_phases(request):
>       phases = request.config.getoption("--fork")
E       ValueError: no option named 'fork'

p.s. Thank Ori and Danny for reporting it!

How did I fix it

This PR fixes this special case so that it will run tests against ALL_FORKS even if the directory path is unset.

Also, this PR adds _validate_fork_name to verify if the given fork name is valid.

Note: if anyone uses custom options without target file or directory path, it will raise errors. e.g., pytest -k test_slots_1 --bls-type milagro is invalid.

✦9 ➜ pytest -k test_slots_1 --bls-type milagro
ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: --bls-type
  inifile: None
  rootdir: /Users/hwwang/ethereum/consensus-specs

[djrtwo]

thank you! minor it