testing: add runIfMain

Once we support imports.meta.url and have a way for testing if the current script is the main script, we want to have a two line snippit of code that will allow any test to be executed as a standalone test deno foo_test.ts rather than running the main entry point. Essentially something like this: https://github.com/tensorflow/tensorflow/blob/c4e61ac1fd0b3b48461e2876b20fe56c09faa2ff/tensorflow/python/lib/io/tf_record_test.py#L484

The most basic form would be something like this:

if (args[1] == imports.meta.url) {
  testMain();
}

But perhaps we can make this shorter by having a function like runIfMain(imports.meta.url)

This came up in: https://github.com/denoland/deno_std/pull/129

[ry]

@bartlomieju Yes - I think if we implement window.location as described in https://github.com/denoland/deno/issues/1750 then we can do

if (imports.meta.url == window.location.toString()) {
  runTests();
}

[bartlomieju]

CC @ry