Reduce Ubuntu and MacOS runners from 4 to 3

The Windows runners are the pytest bottleneck by a margin, so not much
is gained by having the 4th runners. This way we reduce the usage. This
is probably helpful when runners are scarce.

.github/workflows/test.yml

@@ -43,6 +43,8 @@ jobs:
 
  # Prune the indices so that we only run the splits up to the group size
  # defined below in "include". (This is ugly but effective.)
+ - os: ubuntu
+ pytest-split-group-index: 4
  - os: ubuntu
  pytest-split-group-index: 5
  - os: ubuntu
@@ -55,6 +57,8 @@ jobs:
  pytest-split-group-index: 9
  - os: ubuntu
  pytest-split-group-index: 10
+ - os: macos
+ pytest-split-group-index: 4
  - os: macos
  pytest-split-group-index: 5
  - os: macos
@@ -69,9 +73,9 @@ jobs:
  pytest-split-group-index: 10
  include:
  - os: ubuntu
- pytest-split-group-size: 4
+ pytest-split-group-size: 3
  - os: macos
- pytest-split-group-size: 4
+ pytest-split-group-size: 3
  - os: windows
  pytest-split-group-size: 10