Consider moving deno testPerm and unit_test_runner to std library

[bartlomieju]

Once we get support for import() and start working on more concrete test runner I suggest we should consider introducing testPerm function and logic from unit_test_runner to standard library.

Rationale:
Since we have Deno.permissions() API it's crucial to test applications with different set of permission and I believe we should support it as first class citizen.

I decided to open this issue because I hit problem described above (need to test library with and without --allow-env) and don't to want copy runner code just to my repo.

testPerm(perms: TestPermissions) and spawning new Deno process for each combination of discovered permissions is working fine in main repo so we might start out from there and work toward better implementation as needed.

[bartlomieju]

This issue should be revisited after landing test runner #516