TST: Run macosx backends in a subprocess

In the macosx backend, `event_loop_is_running` returns true after
`lazy_init` is called, which happens in the `new` constructor for _any_
class. This means it is "running" as soon as a figure is created, and
this will block any other backend tests.

lib/matplotlib/tests/test_backend_macosx.py

@@ -1,18 +1,18 @@
 import os
+from pathlib import Path
 
 import pytest
 from unittest import mock
 
 import matplotlib as mpl
 import matplotlib.pyplot as plt
-try:
-    from matplotlib.backends import _macosx
-except ImportError:
-    pytest.skip("These are mac only tests", allow_module_level=True)
+from matplotlib.testing import subprocess_run_helper
 
 
-@pytest.mark.backend('macosx')
-def test_cached_renderer():
+_test_timeout = 60
+
+
+def _test_cached_renderer():
     # Make sure that figures have an associated renderer after
     # a fig.canvas.draw() call
     fig = plt.figure(1)
@@ -24,8 +24,14 @@ def test_cached_renderer():
     assert fig.canvas.get_renderer()._renderer is not None
 
 
-@pytest.mark.backend('macosx')
-def test_savefig_rcparam(monkeypatch, tmp_path):
+@pytest.mark.backend('macosx', skip_on_importerror=True)
+def test_cached_renderer():
+    subprocess_run_helper(_test_cached_renderer, timeout=_test_timeout,
+                          extra_env={"MPLBACKEND": "macosx"})
+
+
+def _test_savefig_rcparam():
+    tmp_path = Path(os.environ["TEST_SAVEFIG_PATH"])
 
     def new_choose_save_file(title, directory, filename):
         # Replacement function instead of opening a GUI window
@@ -34,9 +40,10 @@ def new_choose_save_file(title, directory, filename):
         os.makedirs(f"{directory}/test")
         return f"{directory}/test/{filename}"
 
-    monkeypatch.setattr(_macosx, "choose_save_file", new_choose_save_file)
     fig = plt.figure()
-    with mpl.rc_context({"savefig.directory": tmp_path}):
+    with (mock.patch("matplotlib.backends._macosx.choose_save_file",
+                     new_choose_save_file),
+          mpl.rc_context({"savefig.directory": tmp_path})):
         fig.canvas.toolbar.save_figure()
         # Check the saved location got created
         save_file = f"{tmp_path}/test/{fig.canvas.get_default_filename()}"
@@ -47,14 +54,20 @@ def new_choose_save_file(title, directory, filename):
         assert mpl.rcParams["savefig.directory"] == f"{tmp_path}/test"
 
 
-@pytest.mark.backend('macosx')
+@pytest.mark.backend('macosx', skip_on_importerror=True)
+def test_savefig_rcparam(tmp_path):
+    subprocess_run_helper(
+        _test_savefig_rcparam, timeout=_test_timeout,
+        extra_env={"MPLBACKEND": "macosx", "TEST_SAVEFIG_PATH": tmp_path})
+
+
+@pytest.mark.backend('macosx', skip_on_importerror=True)
 def test_ipython():
     from matplotlib.testing import ipython_in_subprocess
     ipython_in_subprocess("osx", {(8, 24): "macosx", (7, 0): "MacOSX"})
 
 
-@pytest.mark.backend('macosx')
-def test_save_figure_return():
+def _test_save_figure_return():
     fig, ax = plt.subplots()
     ax.imshow([[1]])
     prop = "matplotlib.backends._macosx.choose_save_file"
@@ -65,3 +78,9 @@ def test_save_figure_return():
     with mock.patch(prop, return_value=None):
         fname = fig.canvas.manager.toolbar.save_figure()
         assert fname is None
+
+
+@pytest.mark.backend('macosx', skip_on_importerror=True)
+def test_save_figure_return():
+    subprocess_run_helper(_test_save_figure_return, timeout=_test_timeout,
+                          extra_env={"MPLBACKEND": "macosx"})