Trac #33521: doctesting with pytest fails on system install of sage

pytest assumes the directory containing the code to be tested is
writable. This is not currently the case on sage-on-distro were most of
the time testing is done after installing on the system.

Here is a typical result
{{{
============================= test session starts
==============================
platform linux -- Python 3.10.2, pytest-7.0.1, pluggy-1.0.0
rootdir: /usr
plugins: hypothesis-6.38.0
collected 0 items

=============================== warnings summary
===============================
../../usr/lib/python3.10/site-packages/_pytest/cacheprovider.py:433
  /usr/lib/python3.10/site-packages/_pytest/cacheprovider.py:433:
PytestCacheWarning: could not create cache path
/usr/.pytest_cache/v/cache/nodeids
    config.cache.set("cache/nodeids", sorted(self.cached_nodeids))

../../usr/lib/python3.10/site-packages/_pytest/stepwise.py:52
  /usr/lib/python3.10/site-packages/_pytest/stepwise.py:52:
PytestCacheWarning: could not create cache path
/usr/.pytest_cache/v/cache/stepwise
    session.config.cache.set(STEPWISE_CACHE_DIR, [])

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
}}}
pytest can be started with some option to change the location of the
cache directory `-o cache_dir=...`. pytest is currently called from
`sage-runtest` and this is where we may want to apply any changes.

URL: https://trac.sagemath.org/33521
Reported by: fbissey
Ticket author(s): Matthias Koeppe
Reviewer(s): François Bissey

src/bin/sage-runtests

@@ -150,6 +150,14 @@ if __name__ == "__main__":
     DC = DocTestController(args, args.filenames)
     err = DC.run()
 
+    # Trac #33521: Do not run pytest if the pytest configuration is not available.
+    # This happens when the source tree is not available and SAGE_SRC falls back
+    # to SAGE_LIB.
+    from sage.env import SAGE_SRC
+    if not all(os.path.isfile(os.path.join(SAGE_SRC, f))
+               for f in ["conftest.py", "tox.ini"]):
+        sys.exit(err)
+
     try:
         exit_code_pytest = 0
         import pytest
@@ -169,7 +177,7 @@ if __name__ == "__main__":
                 exit_code_pytest = 0
 
     except ModuleNotFoundError:
-        print("Pytest is not installed, skip checking tests that rely on it.")
+        print("pytest is not installed in the venv, skip checking tests that rely on it")
 
     if err == 0:
         sys.exit(exit_code_pytest)