TST: Followup corrections to #28205

Make the changes suggested by @ianthomas23, and also mark the ipython
tests as using their backend. While the backend is already checked for
availability at the top of the respective files, that only checks
whether it can be imported, not whether it can be set as the Matplotlib
backend. Adding the marker causes our pytest configuration to actually
check and skip the test if unavailable (e.g., on Linux without
`(WAYLAND_)DISPLAY` set fails to set an interactive backend).

lib/matplotlib/testing/__init__.py

@@ -210,4 +210,4 @@ def ipython_in_subprocess(requested_backend_or_gui_framework, all_expected_backe
         capture_output=True,
     )
 
-    assert proc.stdout.strip() == f"Out[1]: '{expected_backend}'"
+    assert proc.stdout.strip().endswith(f"'{expected_backend}'")

lib/matplotlib/tests/test_backend_inline.py

@@ -1,6 +1,7 @@
 import os
 from pathlib import Path
 from tempfile import TemporaryDirectory
+import sys
 
 import pytest
 
@@ -12,6 +13,7 @@
 pytest.importorskip('matplotlib_inline')
 
 
+@pytest.mark.skipif(sys.version_info[:2] <= (3, 9), reason="Requires Python 3.10+")
 def test_ipynb():
     nb_path = Path(__file__).parent / 'test_inline_01.ipynb'
 

lib/matplotlib/tests/test_backend_macosx.py

@@ -46,6 +46,7 @@ def new_choose_save_file(title, directory, filename):
         assert mpl.rcParams["savefig.directory"] == f"{tmp_path}/test"
 
 
+@pytest.mark.backend('macosx')
 def test_ipython():
     from matplotlib.testing import ipython_in_subprocess
     ipython_in_subprocess("osx", {(8, 24): "macosx", (7, 0): "MacOSX"})

lib/matplotlib/tests/test_backend_qt.py

@@ -374,6 +374,7 @@ def custom_handler(signum, frame):
         signal.signal(signal.SIGINT, original_handler)
 
 
+@pytest.mark.backend('QtAgg', skip_on_importerror=True)
 def test_ipython():
     from matplotlib.testing import ipython_in_subprocess
     ipython_in_subprocess("qt", {(8, 24): "qtagg", (8, 15): "QtAgg", (7, 0): "Qt5Agg"})