add CI build

Author: jorisvandenbossche

.github/workflows/unit-tests.yml

@@ -69,6 +69,10 @@ jobs:
             env_file: actions-311.yaml
             pattern: "not slow and not network and not single_cpu"
             pandas_copy_on_write: "1"
+          - name: "Copy-on-Write (warnings)"
+            env_file: actions-311.yaml
+            pattern: "not slow and not network and not single_cpu"
+            pandas_copy_on_write: "warn"
           - name: "Pypy"
             env_file: actions-pypy-39.yaml
             pattern: "not slow and not network and not single_cpu"