Cleaned up more TODOs.

[BigBlueHat]

• Avoid using .exist.
• Keep should var around, but fix linting.
• Add VP id test.
• Add VP type check.
• Anchor text links to VP section.
• Rename presentation-derive tests to enveloped.
• Update envelope fixtures to use objects.
• Turn on VP enveloped VC tests.
• Move enveloped tests into enveloped section.
• Add failure test of string VC in VP.

[BigBlueHat]

@aljones15 I removed the commit that was avoiding .exist() use, so this is back to how it was--which was simpler.

[BigBlueHat]

@aljones15 @PatStLouis could use a re-review. Rebased and removed those questionable .exist() replacements.