This patch introduces a tiny test runner to run puppeteer tests.
The test runner is self-container and allows parallel (wrt IO) test execution.
It will also allow us to split tests into multiple files if necessary.
Comparing to the jasmine, the testrunner supports parallel execution, properly
handles "unhandled promise rejection" event and signals.
Comparing to ava/jest, the testrunner doesn't run multiple node processes,
which makes it simpler but sufficient for our goals.
