As described in #1033: Currently, developers in environments without gnu
coreutils (notably: macOS) get an extremely confusing error message when
they run `yarn test`. This commit ensures that such users instead get a
helpful message with a link to the fix.
Follows @wchargin's proposed approach for fixing #1033, so that Mac
developers get clear guidance on how to get their development
environment working properly.
Expected behavior is that `yarn test` passes when GNU coreutils are
present, and fails with the following message when they are not:
Fixes#1033
```
FAIL check-gnu-coreutils
Exit code: 1
Contents of stdout:
/home/dandelion/git/sc/sourcecred
Contents of stderr:
Error: Your environment does not provide GNU coreutils
You're likely developing on macOS.
Please see the following link for a fix:
https://github.com/sourcecred/sourcecred/issues/698#issuecomment-417202213
```
Test plan:
Verify that on a machine with gnu coreutils present, the command passes
and does not print irrelevant output.
Verify that on a machine without gnu coreutils available, the command
fails, and prints exactly the error message expected, without any extra
output related to the test's implementation.
Paired-with: @anthrocypher
eac0a3ebee