pytester: use temporary HOME with spawn

[blueyed]

Followup to #4956.

[blueyed]

Refactored it, which also allows for adjusting it via testdir._env_run_update.

[blueyed]

This would allow for the following now, which works around pdb++ hijacking pdb and adding source code highlighting, which fails some (3) tests currently. Not sure if it should be added though, given that it is rather specific (pdbpp being installed), and tests could be adjusted to cope with it (i.e. loser/other assertions).

diff --git i/testing/test_pdb.py w/testing/test_pdb.py
index c0ba7159b..81e455b13 100644
--- i/testing/test_pdb.py
+++ w/testing/test_pdb.py
@@ -76,6 +76,13 @@ def set_trace(self, frame):
     del _pytest._CustomDebugger


+@pytest.fixture(autouse=True)
+def pdb_env(request):
+    if "testdir" in request.funcargnames:
+        testdir = request.getfixturevalue("testdir")
+        testdir._env_run_update["PDBPP_HIJACK_PDB"] = "0"
+
+
 class TestPDB(object):
     @pytest.fixture
     def pdblist(self, request):

[blueyed]

Note that the above fixture is different from setting PDBPP_HIJACK_PDB=0 before running pytest: it would still use pdbpp then in the outer tests.

[nicoddemus]

This would allow for the following now, which works around pdb++ hijacking pdb and adding source code highlighting, which fails some (3) tests currently. Not sure if it should be added though, given that it is rather specific (pdbpp being installed), and tests could be adjusted to cope with it (i.e. loser/other assertions).

I'm fine if this helps people with pdb++ installed to run the pytest suite (such as yourself), just please make sure to add an informative docstring to the fixture though.

Also feel free to merge this now if you prefer to introduce this fixture in a future PR instead.

[blueyed]

Test was skipped - needs #5313.