* refactor: preliminary cleanup of Deno.runTests()
* Change time measurement to use new Date() instead of
performance.now(). Because there is no guarantee that tests are
run with "--allow-hr" using new Date() guarantees higher
precision of 1ms instead of 2ms.
* Support String type filter in "skip" and "only".
* Split "exitOnFail" into "exitOnFail" and "failFast".
Former tells if "runTests()" should exit with code 1 on test
failure, while latter tells if "runTests()" should stop
running tests on first failure.
* Use "defer" to wait for unhandled promise rejection - this bit
is funky and doesn't seem right, but for now it's just a rewrite
from using "setTimeout". Intended to be fixed in later commits.
* Remove global "__DENO_TEST_REGISTRY", don't expose list of
registered tests (to be addressed in follow up commits)
* Remove arbitrary slow test threshold; use uniform coloring
instead
* 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
