Refactor _test into integration_tests

[davidmorgan]

Cases moved to integration_tests:

• Post process builders
• e2e invalidation with source resolution
• serving indexes, etags
• web compilers: dart2js, DDC, dart2wasm: successful builds, basic check on output

Deleted:

• a lot of the compiler e2e tests were just checking config handling, which is already covered elsewhere
• behaviour of tests running in Chrome; this is more about package:test and the compilers
• script generation golden test

CI config tweaks: rename "unit_test" to "test", move leak check test into the same group.

[github-actions]

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