Have cli tests set env variable OMP_NUM_THREADS to 1.

This commit adds an auto-used fixture for all cli tests, that sets the environment variable for numpy multithreading `OMP_NUM_THREADS` to 1.
This might have been the cause of those tests hanging, as numpy uses all resources available if the env variable is not explicitly set.

tests/ui_tests/cli/conftest.py

@@ -0,0 +1,16 @@
+import os
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def reduce_omp_num_threads_count():
+    old_omp_num_threads = os.environ.get("OMP_NUM_THREADS")
+    os.environ["OMP_NUM_THREADS"] = "1"
+
+    yield
+
+    if old_omp_num_threads is None:
+        del os.environ["OMP_NUM_THREADS"]
+    else:
+        os.environ["OMP_NUM_THREADS"] = old_omp_num_threads