test: create more VertexCore tests

[AustinGreen]

Motivation:

Close #187

Modifications:

Completed the TODOs in VertexCore besides the signature functions that will be merged in #208 and integration tests.

Result:

VertexCore test coverage will improve.

[mds1]

Just a few tiny things

[0xrajath]

LGTM. Though I think you should address @mds1 comments first. They're pretty good points

[github-actions]

Coverage after merging austin/vertex-core-followup-tests into main will be

89.57%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src
VertexAccount.sol	100%	100%	100%	100%
VertexCore.sol	91%	87.93%	85%	93.44%	279, 286–287, 287, 287–288, 313, 320–321, 321, 321–322, 427, 499, 503
VertexFactory.sol	100%	100%	100%	100%
VertexLens.sol	100%	100%	100%	100%
VertexPolicy.sol	87.40%	77.78%	90%	88.61%	142, 185–186, 188, 240–241, 246, 251–252, 271, 327, 336, 374
VertexPolicyMetadata.sol	100%	100%	100%	100%
VertexStrategy.sol	97.96%	83.33%	100%	100%	178
src/lib
Checkpoints.sol	79.68%	72.22%	94.44%	80.87%	105, 204, 216–217, 258–260, 260, 260–261, 263, 266, 294–295, 333–335, 337–339, 341–343, 345–347, 349–351, 353–355, 361–362, 380, 389, 53
ERC721NonTransferableMinimalProxy.sol	72.46%	72.73%	72.73%	72.22%	102, 104, 106, 118, 190, 88, 88, 88, 90, 90, 90, 92, 92, 92, 97, 99