tests for coverage

[Fil]

Now that we have test:coverage, it becomes easier to add more tests.

From the starting point of (#1818):

----------------------|---------|----------|---------|---------|
File                  | % Stmts | % Branch | % Funcs | % Lines |
----------------------|---------|----------|---------|---------|
All files             |   95.54 |    89.37 |   92.78 |   95.54 |

Current status:

All files             |   95.66 |    89.65 |    92.8 |   95.66 |

(It doesn’t look like much, but each addition is worth it!)

[mbostock]

I made some changes:

1. I folded the tests into the parent class or function. For example we already had tests for the line mark’s curve option, so I moved these new tests to be adjacent.
2. I changed the test descriptions to match the current pattern (which is mostly method(...arguments) does something).
3. I adopted TypeScript for the tests whenever possible. (We should file an issue for going back and moving all the tests to TypeScript, but we’ll need to use a lot of as any to test the undocumented implementation.)
4. I adopted jsdom.js; the eliminates the need for instantiating JSOM and manually passing the document option to Plot.