Set mplbackend to Agg for fmudesign cli tests

Author: larsevj

tests/fmudesign/test_use_cases.py

@@ -177,8 +177,14 @@ def test_all_input_files(tmpdir, monkeypatch, designfile, verbosity):
 
     # Run the CLI tool (test will fail on non-zero status code)
     verbose = ["--verbose"] * verbosity
+    env = os.environ.copy()
+    env.setdefault("MPLBACKEND", "Agg")
     subprocess.run(
-        ["fmudesign", designfile, *verbose], check=True, capture_output=True, text=True
+        ["fmudesign", designfile, *verbose],
+        check=True,
+        capture_output=True,
+        text=True,
+        env=env,
     )