shard unit tests in CI for faster CI runs (#5572)

* split main unit tests across 2 processes to increase ci speed

* use 3 shards for regular tests and 2 shards for slow tests

* explicitly define each shard

the strategy.job_total figure can easily change, which would cause pretty big
problems with our tests. if we manually define the entire shard piece instead
of trying to calculate it, this makes the job more robust to future changes.

.github/workflows/ci.yml

@@ -42,6 +42,9 @@ jobs:
   test:
     name: Test
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        shard: [1/3, 2/3, 3/3]
 
     steps:
       - name: Check out Git repository
@@ -63,7 +66,7 @@ jobs:
         run: yarn --non-interactive --frozen-lockfile
 
       - name: Run tests
-        run: yarn test:coverage --maxWorkers=2 --workerIdleMemoryLimit=2000MB
+        run: yarn test:coverage --maxWorkers=2 --workerIdleMemoryLimit=2000MB --shard=${{ matrix.shard }}
 
       - name: Check for missing fixtures
         run: |
@@ -84,6 +87,9 @@ jobs:
   testslow:
     name: Slow Tests
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        shard: [1/2, 2/2]
 
     steps:
       - name: Check out Git repository
@@ -105,7 +111,7 @@ jobs:
         run: yarn --non-interactive --frozen-lockfile
 
       - name: Run slow tests & coverage
-        run: yarn test:slow:coverage --maxWorkers=2 --workerIdleMemoryLimit=2000MB
+        run: yarn test:slow:coverage --maxWorkers=2 --workerIdleMemoryLimit=2000MB --shard=${{ matrix.shard }}
 
       - name: Check for missing fixtures
         run: |