Rewrite "testPerm" helper function used for testing of internal
runtime code. It's been renamed to "unitTest" and provides API that
is extensible in the future by accepting optional "UnitTestOptions"
argument. "test" helper was also removed and replaced by
overloaded version of "unitTest" that takes only function argument.
"UnitTestOptions" currently supports "perms" and "skip"
options, where former works exactly as first argument to "testPerm"
did, while the latter allows to conditionally skip tests.
* 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
* rewrite test runner in Rust
* migrate "test" and "runTests" functions from std to "Deno" namespace
* use "Deno.test()" to run internal JS unit tests
* remove std downloads for Deno subcommands
