Adapt golden testing framework from pub client

[sigurdm]

(adapted from https://github.com/dart-lang/pub/blob/master/test/golden_file.dart)

With this:

• golden files gets their name from the running test automatically
• there is a concept of multiple sections in a golden file
• you can update the golden files by running the tests with _WRITE_GOLDENS=TRUE.
• You can run a command, and have the command, stdout, stderr and exit code all included in the golden file.
  • and have specific expectation machers on those at the same time
• missing golden files cause an error on CI.

[isoos]

While the filenames with spaces are probably a non-issue, shouldn't we rather have - or _ instead of spaces?