Actually split :server:x-of-y: tests (#858)

The split-tests.js script did not do anything because the TEST_SPLITS
variable was empty, because the TESTS variable was not being initialized
from the matrix.tests value.

This resulted in the entire :server: test suite being run twice.

.github/workflows/main.yml

@@ -98,6 +98,7 @@ jobs:
           export TEST_SPLITS=$(echo $TESTS | sed "s/.*:server-\([^:]*\).*/\1/")
           MOCHA_WEBDRIVER_HEADLESS=1 yarn run test:server
         env:
+          TESTS: ${{ matrix.tests }}
           GRIST_DOCS_MINIO_ACCESS_KEY: administrator
           GRIST_DOCS_MINIO_SECRET_KEY: administrator
           TEST_REDIS_URL: "redis://localhost/11"