Refactor integration tests

[davidmorgan]

Refactor integration_tests and some tweaks to _tests and other tests:

• Some integration_tests were not fully end to end, they wrote their own build.dart script which called the build_runner run method bypassing the build_runner bootstrap code; this is the main reason for doing the rewrite now as this is awkward for the bootstrap rewrite currently in progress. Make them all us dart run build_runner so they are actually end to end.
• Remove tests from _tests that are covered in integration_tests: Re-snapshots if there is no asset graph, exception_handling_test.dart
• Remove help_test, this is testing configuration and not really worth an e2e test.
• Merge test cases from build_script_generate_test into integration_tests as they are e2e tests
• Most of build_integration_test.dart is replaced by equivalent tests, except: "checking for existing outputs works with deleted intermediate outputs" is delete, it's covered sufficiently by smaller tests
• symlinks_test is deleted because the results tested are what happens anyway with symlinked files, there is no supporting implementation
• some tests that were checking invalidation with serve now check invalidation with watch instead because watch is much more widely used and the implementation is the same

Update the CI configs:

• unify on SDK version dev, there doesn't seem to be a lot of value in running on different SDKs
• run small tests only on Linux, start running build_runner integration tests on Windows
• run most of the build_runner integration tests along with the unit tests and in parallel because they are pretty small + fast; leave two tests in the "integration tests" bucket that are slow and could use a rewrite, and leave two tests for the "serve" command in the "integration tests" bucket because they need to pick an open port before they can run in parallel; add TODOs to follow up

Further follow-up work:

• See if the remainder of the _test e2e tests can be replaced by integration_tests, mostly this looks like covering web builds

[github-actions]

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