tests: update test-util.py for modern attrs package

When updating to 22.1.0, this test started failing:

    Traceback (most recent call last):
      File "/tmp/mercurial-ci/tests/test-util.py", line 53, in <module>
        _start_default = (util.timedcmstats.start.default, 'factory')
    AttributeError: type object 'timedcmstats' has no attribute 'start'

Poking around in `hg debugshell`, the attribute is indeed missing, but looks to
be attached to `__attrs_attrs__` in both the currently vendored and the modern
version of attrs.  The old attrs packages will print the same for both accesses,
so fingers crossed...

    >>> print((util.timedcmstats.start.default, 'factory'))
    (Factory(factory=<function timedcmstats.<lambda> at 0x000001EFDF0F21F0>, takes_self=False), 'factory')

    >>> print((util.timedcmstats.__attrs_attrs__.start.default, 'factory'))
    (Factory(factory=<function timedcmstats.<lambda> at 0x000001EFDF0F21F0>, takes_self=False), 'factory')

tests/test-util.py

@@ -50,7 +50,7 b' def mocktimer(incr=0.1, *additional_targ'
 
 # attr.s default factory for util.timedstats.start binds the timer we
 # need to mock out.
-_start_default = (util.timedcmstats.start.default, 'factory')
+_start_default = (util.timedcmstats.__attrs_attrs__.start.default, 'factory')
 
 
 @contextlib.contextmanager