[FIX] runbot_merge: testsuite compatibility with pytest-xdist

randomise the name of the repositories created so they don't collide
and lead to odd results when running concurrent test cases which
specify the same repo name (a common property).

As a result, ignore the "no delete" flag for creation: there should be
no way to land on a pre-existing repo name even if we didn't clean
them up.

Also stagger the check of a running ngrok process: when pytest starts
its worker processes, all workers will run the tunnel fixture, and
since the ngrok process takes some time to get into a stable run state
chances are multiple workers will fail to connect and try to start
ngrok concurrently, which blows up as ngrok just kills the extra
processes instead of merging / proxying into an existing session. A
proper lockfile would probably be better but...

Fixes #297

conftest.py

@@ -45,6 +45,8 @@ import configparser
 import copy
 import itertools
 import logging
+import os
+import random
 import re
 import socket
 import subprocess
@@ -149,6 +151,8 @@ def tunnel(pytestconfig, port):
         addr = 'localhost:%d' % port
         # if ngrok is not running, start it
         try:
+            # FIXME: use lockfile instead
+            time.sleep(random.randint(1, 10))
             # FIXME: use config file so we can set web_addr to something else
             #        than localhost:4040 (otherwise we can't disambiguate
             #        between the ngrok we started and an ngrok started by
@@ -321,17 +325,9 @@ def make_repo(request, config, tunnel, users):
 
     repos = []
     def repomaker(name):
+        name = 'ignore_%s_%s' % (name, base64.b64encode(os.urandom(6), b'-_').decode())
         fullname = '{}/{}'.format(owner, name)
         repo_url = 'https://api.github.com/repos/{}'.format(fullname)
-        if request.config.getoption('--no-delete'):
-            if github.head(repo_url).ok:
-                pytest.skip("Repository {} already exists".format(fullname))
-        else:
-            # just try to delete the repo, we don't really care
-            if github.delete(repo_url).ok:
-                # if we did delete a repo, wait a bit as gh might need to
-                # propagate the thing?
-                time.sleep(30)
 
         # create repo
         r = github.post(endpoint, json={