tests: add util to run a function in separate process

This allows our tests to run in an isolated environment. For tests taht implicitly depend on import behaviour, this can prevent side-effects.

The function should only be used for tests.

tests/test_dangerously_run_function_in_subprocess.py

@@ -0,0 +1,57 @@
+from tests.dangerously_run_function_in_subprocess import dangerously_run_function_in_subprocess
+
+
+def test_simple_function():
+    def test_func():
+        print("Hello, Test!")
+
+    stdout, stderr, returncode = dangerously_run_function_in_subprocess(test_func)
+
+    assert returncode == 0
+    assert stdout.strip() == "Hello, Test!"
+    assert stderr == ""
+
+
+def test_function_with_error():
+    def test_func():
+        raise ValueError("This is an error")
+
+    _stdout, stderr, returncode = dangerously_run_function_in_subprocess(test_func)
+
+    assert returncode != 0  # Should fail
+    assert "ValueError: This is an error" in stderr
+
+
+def test_function_with_imports():
+    def test_func():
+        import math
+
+        print(math.sqrt(4))
+
+    stdout, stderr, returncode = dangerously_run_function_in_subprocess(test_func)
+
+    assert returncode == 0
+    assert stdout.strip() == "2.0"
+    assert stderr == ""
+
+
+def test_function_with_sys_exit():
+    def test_func():
+        import sys
+
+        sys.exit(42)
+
+    _stdout, _stderr, returncode = dangerously_run_function_in_subprocess(test_func)
+
+    assert returncode == 42  # Should return the custom exit code
+
+
+def test_function_with_closure():
+    foo = "bar"
+
+    def test_func():
+        print(foo)
+
+    _stdout, _stderr, returncode = dangerously_run_function_in_subprocess(test_func)
+
+    assert returncode == 1  # Should fail because of closure