Don't clobber screenshot dir when running with xdist

Ohhh! I didn't realize the e2e tests where running in parallel already.
Interesting.

Anyhow, use the builtin `filelock` module to make sure different test
processes in the same session don't re-create the screenshot directory.

This is based on advice here: https://pytest-xdist.readthedocs.io/en/latest/how-to.html#making-session-scoped-fixtures-execute-only-once

I'm saving the lock object to the session stash because it seems the
lock is released when the FileLock object is destroyed. So this ties
it's lifecycle to the test session lifecycle, with xdist hopefully all
the tests processes live for the whole run.

tests/end2end/fixtures/quteprocess.py

@@ -17,6 +17,7 @@ import contextlib
 import itertools
 import collections
 import json
+import filelock
 
 import yaml
 import pytest
@@ -881,11 +882,26 @@ class QuteProc(testprocess.Process):
 
         temp_path = os.environ.get("RUNNER_TEMP", tempfile.gettempdir())
         screenshot_path = pathlib.Path(temp_path) / "pytest-screenshots"
+
+        lock = filelock.FileLock(screenshot_path / ".pytest.lock")
         if screenshot_path.exists():
-            shutil.rmtree(screenshot_path)
-        screenshot_path.mkdir()
+            # Clean and re-create dir for a new run. Except if the lock file
+            # is being held then we are running in parallel.
+            try:
+                lock.acquire(blocking=False)
+            except filelock.Timeout:
+                pass
+            else:
+                lock.release()
+                shutil.rmtree(screenshot_path)
+                screenshot_path.mkdir()
+                lock.acquire()
+        else:
+            screenshot_path.mkdir()
+            lock.acquire()
 
         self.request.session.stash["screenshot_path"] = screenshot_path
+        self.request.session.stash["screenshot_lock"] = lock
         return screenshot_path
 
     def _take_x11_screenshot_of_failed_test(self):
@@ -908,10 +924,9 @@ class QuteProc(testprocess.Process):
             fname = fname.replace(char, "_")
 
         # TODO:
-        # 1. Keep old directories around?
-        # 2. Will different runs in parallel in CI clobber the folder? Might
-        #    have to put them in subdirs with the process ID if so.
-        # 4. Log a "screenshot saved to ..." message?
+        # 1. Log a "screenshot saved to ..." message so that people know where
+        #    to go look for them when running locally? Using pytest-print? Or
+        #    add an FYI to the report summary?
         fpath = self._get_x11_screenshot_directory() / fname
         img.save(fpath)