Re-factor test fixture loader

[gurukamath]

          Inserting by index looks extremely brittle. Could we do something like:

from dataclass import fields

@dataclass
class Foobar:
    nonce: Optional[Uint]
    gasLimit: Optional[Uint]
    # ...

    def to_list(self) -> List[?]:
        parameters = []
        for name in fields(self):
            value = getattr(self, name)
            if value is None:
                continue
            parameters.append(value)

Or is that overkill?

_Originally posted by @SamWilsn in https://github.com/ethereum/execution-specs/pull/867#discussion_r1475180953_

[gurukamath]

Tracking issue before updating to the latest tests

[gurukamath]

Latest tests have been updated in #908