Rewrites "cli/js/unit_test_runner.ts" to communicate with spawned subprocesses
using TCP socket.
* Rewrite "Deno.runTests()" by factoring out testing logic to private "TestApi"
class. "TestApi" implements "AsyncIterator" that yields "TestEvent"s,
which is an interface for different types of event occuring during running
tests.
* Add "reporter" argument to "Deno.runTests()" to allow users to provide custom
reporting mechanism for tests. It's represented by "TestReporter" interface,
that implements hook functions for each type of "TestEvent". If "reporter"
is not provided then default console reporting is used (via
"ConsoleReporter").
* Change how "unit_test_runner" communicates with spawned suprocesses. Instead
of parsing text data from child's stdout, a TCP socket is created and used
for communication. "unit_test_runner" can run in either "master" or "worker"
mode. Former is responsible for test discovery and establishing needed
permission combinations; while latter (that is spawned by "master") executes
tests that match given permission set.
* Use "SocketReporter" that implements "TestReporter" interface to send output
of tests to "master" process. Data is sent as stringified JSON and then
parsed by "master" as structured data. "master" applies it's own reporting
logic to output tests to console (by reusing default "ConsoleReporter").
All Deno runtime test files were moved to cli/js/tests/ directory.
It makes a clear distinction that cli/js/tests/ contains code
that is run under Deno runtime as opposed to code in cli/js/ which
is used to create bundle and snapshot with "deno_typescript".
