Add more package:test (removing usages of package:litetest)

[matanlurey]

... also removed a few TODOs, deleted a proof of concept directory.

Work towards flutter/flutter#133569.

[loic-sharma]

LGTM

[gaaclarke]

I didn't see this PR and started migrating one package myself. I think you are running into the same problem I ran into here: #54909

When invoking litetest it looks like we didn't have asserts turned on so tests would expect that we'd get a StateError, but instead we'd get an assertion error when executing with dart test. I solved it by removing the assertion in the skia gold client since the StateError is thrown in the same exact case after the assert.

Example failure:

00:00 �[32m+12�[0m�[31m -1�[0m: test/skia_gold_client_test.dart: fails if GOLDCTL is not set �[1m�[31m[E]�[0m�[0m
  'package:skia_gold_client/skia_gold_client.dart': Failed assertion: line 193 pos 7: 'isAvailable(environment: _environment)': Trying to use `goldctl` in an environment where it is not available
  dart:core                                             _AssertionError._throwNew
  package:skia_gold_client/skia_gold_client.dart 193:7  SkiaGoldClient._goldctl
  package:skia_gold_client/skia_gold_client.dart 212:7  SkiaGoldClient.auth
  test/skia_gold_client_test.dart 85:22                 main.<fn>