Improve output when running commands across the whole monorepo

Because `yarn workspaces foreach` shows output from many commands, if you run e.g. `yarn test` and there is one package whose tests fails, you have to painstakingly scroll up and rake over the output with a fine-toothed comb. We should make this output easier to read (especially for tests).

Start by reproducing the output from `yarn workspaces foreach` with a failing `yarn test` run across the monorepo. No files or tests are named; the work is done when failures, especially test failures, can be found and understood without painstakingly searching the combined output.

Written by the indexing model from the issue text.