Testing Framework for Hot Reload/Restart Tests for DDC

<!--- TODO(srujzs): Document this package more. For now, document some conventions that may not be obvious. --->

This package contains utilities to test multiple generations of files in order to validate hot reload and hot restart.

Test File Conventions

• Different generations of files have a generation number after the file name e.g. file_name.1.dart is the generation 1-version of the file.
• If a generation file is intended to be rejected, it should contain .reject in the file name. The config.json file should contain a key "expectedErrors" with its value being a map of generation number string to the error string.
• If a generation does a hot restart instead of a reload, it should contain .restart in every file name in the same generation.
• It is an error to specify both .reject and .restart.