Separate tests into their own directory

[twpayne]

Currently tests are in src/index.spec.ts, which is the same directory as the source code. The TypeScript compiler also currently generates several files from this in the dist/ directory, which are unwanted:

dist/index.spec.js
dist/index.spec.js.map
dist/types/index.spec.d.ts
dist/types/index.spec.d.ts.map

It would be nice to move the tests into their own subdirectory, say tests/, and not have these test-related files generated in dist/.

Note that the tests also run both the TypeScript tests and the JavaScript tests generated from the TypeScript tests:

$ npm test

> @twpayne/xcscore@1.0.2 test /Users/twp/src/github.com/twpayne/xcscore-js
> jest

 PASS  src/index.spec.ts
 PASS  dist/index.spec.js

Test Suites: 2 passed, 2 total
Tests:       70 passed, 70 total
Snapshots:   0 total
Time:        2.077s
Ran all test suites.

Note that both src/index.spec.ts and dist/index.spec.js are both run. It would be nice if separating the tests also meant that only src/index.spec.ts was run.