Add tests for is_data_available in fork choice

[hwwhww]

We can inject is_data_available output in the way we hacked get_pow_block in on_merge_block tests:

consensus-specs/tests/core/pyspec/eth2spec/test/bellatrix/fork_choice/test_on_merge_block.py

Lines 29 to 50 in e9a7a6d

	def with_pow_block_patch(spec, blocks, func):
	def get_pow_block(hash: spec.Bytes32) -> spec.PowBlock:
	for block in blocks:
	if block.block_hash == hash:
	return block
	raise BlockNotFoundException()
	get_pow_block_backup = spec.get_pow_block
	spec.get_pow_block = get_pow_block
	class AtomicBoolean():
	value = False
	is_called = AtomicBoolean()
	def wrap(flag: AtomicBoolean):
	yield from func()
	flag.value = True
	try:
	yield from wrap(is_called)
	finally:
	spec.get_pow_block = get_pow_block_backup
	assert is_called.value

TODOs

• Update fork-choice test format
• Add test cases

[hwwhww]

closing in favor of #3463