Cleanup parser specs

[FnControlOption]

Note: Commit ea79701 may be controversial. I slightly prefer explicit test cases over loops, but I have no strong feelings either way!

[straight-shoota]

I too prefer unrolled loops for explicitness and grepability.

The loops however do provide value in grouping: They express that all examples are functionally equivalent, just testing different values.
This notion gets lost when they're just unrolled into the long list of examples.

Some sense of togetherness could be established by grouping the examples via describe. This would also assign a description which further helps understanding the purpose. We're missing that far too much in these massive spec files.

The other aspect of functional equivalence has particular relevance for more complex examples: Generalizing the implementation helps keep the code clean and easy to read. For example, extract the shared plumbing into a spec helper method and let the unrolled loop items call that explicitly. Perhaps that could be a strategy to explore for further enhancements of the more complex loops not touched in thie PR.
Of course this mechanism reduces grepability again if the tested source code is constructed in a helper method. It's all about balance.

# loop
%[a b c].each do |value|
  it "#{value} foo" do
    assert_fooing(value)
  end
end

# unrolled with helper
private def it_foos(value)
  it "#{value}.foo" do
    assert_fooing(value)
  end
end

it_foos "a"
it_foos "b"
it_foos "c"

For this PR I'd like to have the former loops grouped into describe blocks.
Unrolling further loops could be left for follow-ups.

[ysbaddaden]

To be honest, I fail to see the benefit of unrolling the loops 🤷

[straight-shoota]

Benefits:

• Better location reporting. With the loop variant, you only get a failure location that points to the generic loop code. No indication of which value it was.
• Better readability. It's super obvious what it_parses ":foo!", "foo!".symbol does. Try to figure that out from

  value = symbol[1, symbol.size - 1]
  value = value[1, value.size - 2] if value.starts_with?('"')
  it_parses symbol, value.symbol

• Better grepability. Wanna look at examples involving the equality operator? Grep for def ==. Only you wouldn't find the loop examples because they generate the code at runtime.