This PR brings assertOps and assertResources sanitizers to Deno.test() API.
assertOps checks that test doesn't leak async ops, ie. there are no unresolved
promises originating from Deno APIs. Enabled by default, can be disabled using
Deno.TestDefinition.disableOpSanitizer.
assertResources checks that test doesn't leak resources, ie. all resources used
in test are closed. For example; if a file is opened during a test case it must be
explicitly closed before test case finishes. It's most useful for asynchronous
generators. Enabled by default, can be disabled using
Deno.TestDefinition.disableResourceSanitizer.
We've used those sanitizers in internal runtime tests and it proved very useful in
surfacing incorrect tests which resulted in interference between the tests.
All tests have been sanitized.
Closes #4208
* My original implementation of `fs.appendFile` used an async API, which, though
it would work fine as a polyfill, wasn't an exact match with the Node API. This PR
reworks that API to mimic the Node API fully as a synchronous void function with
an async internal implementation.
* Refactor move of other internal fs `dirent` and `dir` classes to the _fs internal
directory.
