MAYSKIP: tests: make examples_test run in parallel (mala-project#475)

See also mala-project#478

Closes mala-project#475

test/examples_test.py

@@ -1,16 +1,28 @@
 """Test whether the examples are still working."""
 import importlib
 import runpy
+import os
 
 import pytest
 
 
+# There is https://github.com/RKrahl/pytest-dependency and
+# https://github.com/pytest-dev/pytest-order but we like to keep in simple
+# here. When running in parallel (pytest-xdist), we execute ex01 at most twice,
+# which is OK.
+def check_and_create_ex01_artifact():
+    if not os.path.exists("be_model.zip"):
+        print("Running example basic/ex01 in preparation another example")
+        runpy.run_path("../examples/basic/ex01_train_network.py")
+
+
 @pytest.mark.examples
 class TestExamples:
     def test_basic_ex01(self):
         runpy.run_path("../examples/basic/ex01_train_network.py")
 
     def test_basic_ex02(self):
+        check_and_create_ex01_artifact()
         runpy.run_path("../examples/basic/ex02_test_network.py")
 
     def test_basic_ex03(self):
@@ -20,9 +32,11 @@ def test_basic_ex04(self):
         runpy.run_path("../examples/basic/ex04_hyperparameter_optimization.py")
 
     def test_basic_ex05(self):
+        check_and_create_ex01_artifact()
         runpy.run_path("../examples/basic/ex05_run_predictions.py")
 
     def test_basic_ex06(self):
+        check_and_create_ex01_artifact()
         runpy.run_path("../examples/basic/ex06_ase_calculator.py")
 
     def test_advanced_ex01(self):