test: Restructure & improve npm test scripts (#87)

Tests for `test/assert.js` need to run before the vows test suite, not as part of them.
- Rename `test/assert.vows.js` to `test/assert.vows.checks.js`
- Script `test:assert` only runs those tests
- Script `test:assert:strict` runs them with strict assertions
- Script `test:unit` and `test:vows` are currently the same
  (until we add another testing framework to the mix)
- Update script `test` to run strict assertions only on `test/assert.vows.checks.js`

In the end `npm [run] test` executes the same amount of tests on CI but it's more easy to run a subset.

Added devDependency [`npm-run-all`](https://www.npmjs.com/package/npm-run-all)

#82
#77 (comment)

Co-authored-by: Christian Bewernitz <coder@karfau.de>
Co-authored-by: Chris Brody <chris.brody+brodybits@gmail.com>