refactor: rewrite deno test, add Deno.test() (#3865)

* 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

cli/js/deno.ts

@@ -111,6 +111,7 @@ export { utimeSync, utime } from "./utime.ts";
 export { version } from "./version.ts";
 export { writeFileSync, writeFile, WriteFileOptions } from "./write_file.ts";
 export const args: string[] = [];
+export { test, runTests } from "./testing.ts";
 
 // These are internal Deno APIs.  We are marking them as internal so they do not
 // appear in the runtime type library.