tests: fix run-tests default values in Test constructor

As far as I can tell, default values are evaluated far earlier on
Python 3.6 than 2.7, meaning we've got to be more careful about when
we read the defaults dictionary for the kwarg defaults. Sigh.

With this change, test-run-tests.t is significantly less of a mess
under 3.6.

tests/run-tests.py

@@ -659,10 +659,10 b' class Test(unittest.TestCase):'
 
     def __init__(self, path, outputdir, tmpdir, keeptmpdir=False,
                  debug=False,
-                 timeout=defaults['timeout'],
-                 startport=defaults['port'], extraconfigopts=None,
+                 timeout=None,
+                 startport=None, extraconfigopts=None,
                  py3kwarnings=False, shell=None, hgcommand=None,
-                 slowtimeout=defaults['slowtimeout'], usechg=False,
+                 slowtimeout=None, usechg=False,
                  useipv6=False):
         """Create a test from parameters.
 
@@ -694,6 +694,12 b' class Test(unittest.TestCase):'
 
         shell is the shell to execute tests in.
         """
+        if timeout is None:
+            timeout = defaults['timeout']
+        if startport is None:
+            startport = defaults['port']
+        if slowtimeout is None:
+            slowtimeout = defaults['slowtimeout']
         self.path = path
         self.bname = os.path.basename(path)
         self.name = _strpath(self.bname)