Remove compare_output_test (#1881)

* Cleanup and move test-only code into the test utils library

* Fix coverage for binary-level tests

* dartfmt

* Adjust timeout factor

* Unconditionally add coverage path environment variable

* Move main up to the top; coverage runs take a bit more time

* Remove compare_output_test

* git attributes for output test removal

Author: jcollins-g

.gitattributes

@@ -3,7 +3,3 @@
 # Don't collapse in github code reviews by default.
 lib/src/version.dart linguist-generated=false
 lib/src/html/resources.g.dart linguist-generated=false
-
-# Do collapse in github code reviews by default.
-testing/test_package_docs/** linguist-generated=true
-testing/test_package_docs_dev/** linguist-generated=true

CONTRIBUTING.md

@@ -23,18 +23,10 @@ yet in the issue tracker, start by opening an issue. Thanks!
 
 1. `grind` is needed to run dartdoc integration tests, see installed via `pub global activate grinder`.
 2. When a change is user-facing, please add a new entry to the [changelog](https://github.com/dart-lang/dartdoc/blob/master/CHANGELOG.md)
-3. Please include a test for your change.  `dartdoc` has both `package:test`-style unittests as well as integration tests.  To run the unittests, use `dart test/all.dart`.  Most changes can be tested via a unittest, but some require modifying the [test_package](https://github.com/dart-lang/dartdoc/tree/master/testing/test_package) and regenerating its docs via `grind update-test-package-docs`.
+3. Please include a test for your change.  `dartdoc` has both `package:test`-style unittests as well as integration tests.  To run the unittests, use `grind test`.
 4. For major changes, run `grind compare-sdk-warnings` and `grind compare-flutter-warnings`, and include the summary results in your pull request.
 5. Be sure to format your Dart code using `dartfmt -w`, otherwise travis will complain.
-6. Because there are generated versions of the dartdoc docs for stable and development versions of Dart,
-   you need to update the docs twice:
-   - Download and install the latest STABLE version of dart from [the Dart website](https://www.dartlang.org/tools/sdk).
-     (It's probably easiest to download a zip file and change your PATH to the extracted location's `bin` directory)
-   - Run `pub run grinder update-test-package-docs` to update the stable docs (in `testing/test_package_docs`).
-   - Download and install the latest DEV version of dart from [the Dart website](https://www.dartlang.org/tools/sdk)
-     (It's probably easiest to download a zip file and change your PATH to the extracted location's `bin` directory)
-   - Run `pub run grinder update-test-package-docs` to update the dev docs (in `testing/test_package_docs_dev`).
-7. Post your change via a pull request for review and integration!
+6. Post your change via a pull request for review and integration!
 
 ## Testing