tests: use () instead of \ to wrap lines

This should auto-format more consistently, and is slightly more
typical Python.

Differential Revision: https://phab.mercurial-scm.org/D5992

tests/run-tests.py

@@ -1408,8 +1408,8 b' class TTest(Test):'
         session = str(uuid.uuid4())
         if PYTHON3:
             session = session.encode('ascii')
-        hgcatapult = os.getenv('HGTESTCATAPULTSERVERPIPE') or \
-            os.getenv('HGCATAPULTSERVERPIPE')
+        hgcatapult = (os.getenv('HGTESTCATAPULTSERVERPIPE') or
+                      os.getenv('HGCATAPULTSERVERPIPE'))
         def toggletrace(cmd=None):
             if not hgcatapult or hgcatapult == os.devnull:
                 return