fix: JSDoc null values being interpreted as strings, and added test README

[adam-coster]

fixes #1373

• Includes a test case for default values of each JSON-compatible type.
• Adds an extendible option argument to the schema-validator testing function, with current options allowing passing valid/invalid samples to confirm that the schema behaves as expected
• Adds a very brief README to the tests/ folder, outlining the basic steps of setting up the kind of test required for this PR (I had to do a lot of project exploration to figure out how to go about setting up a functional test case).

[adam-coster]

@domoritz I'm not sure what to do about the single failing test.

The tests for the changes I made for the associated issue ran successfully.

The failing test is due to a mismatch in the expected vega-lite schema. When I ran tests locally there was a failure related to that same schema, which passed when I ran the tests with the UPDATE option. I included that updated schema with this PR and made sure that the test script passed all tests locally.

[domoritz]

Nuke your node modules maybe?

[adam-coster]

@domoritz I've updated the PR to only include the commits that change the files related to the reported issue. Tests should now pass.

It looks like vega-lite was bumped to 5.5.0 right around when I originally forked the project, so the prior issues must have been caused by something going awry when I merged from upstream after that.

[adam-coster]

@domoritz I've added a commit with the requested changes to the test/README.md file