Add assertions on consumption behavior to parser spec tests

[chtenb]

Description of the change
The consumption behavior of our parser functions feels under specified and under documented. I think we should pay some more attention to this, compare the consumption behaviors of other libraries and assert these behaviors in tests. See also the discussion in #95

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[garyb]

Seems like a good idea to me 👍 purescript-parsing would probably benefit from similar treatment too, we could compare any differences then for places they overlap, and hopefully eliminate them.

I'm also curious about the performance difference between the two these days. But that's another project entirely.

[jamesdbrock]

It is definitely true in purescript-parsing also that “consumption behavior of our parser functions feels under specified and under documented.”