Test-runner: replace ava with lab

[feugy]

Ava is slow, lab is simpler and more reliable (it found issued ava didn't reported.
Power-assert remains assertion library.
Some uncovered path were found.

Trade-offs are:

• necessity to distinguish client (use browser-env) and server (node env) to test SSR
• no more parallelism

[coveralls]

Coverage increased (+6.5%) to 98.486% when pulling 9609c8e on use-lab into 6142c15 on master.