* add "assertOps" test assertion which makes sure test case
is not "leaking" ops - ie. after test finishes there are no
pending async ops
* apply "assertOps" to all tests in "cli/js/"
* fix numerous tests leaking ops
* document problem with edge case in "clearInterval"
and "clearTimeout" implementation where they
may leak async ops
* move "cli/js/worker_test.ts" to "cli/tests/worker_test.ts" and
run as integration test; workers leak ops because of missing
"terminate" implementation
This avoids a crash when the Deno process has been running for
2**32 ms (about 50 days). Additionaly, time complexity of finding which
timer is due to fire next is reduced from from O(n) to O(log n).
