They aren't descriptors? They certainly aren't normal methods:
>>> class foo(object):
...   def __str__(self): return "foo"
... 
>>> f = foo()
>>> f.__str__ = lambda: "bar"
>>> str(f)
'foo'

I *thought* the mechanism by which they can only be replaced by altering the class *was* descriptors, but I gladly seek better info!

_testMethodName is overloaded though: its both 'what is the test method' and 'part of the description or id of the test', so resetting it would be - at best - risky when only changing the description or id.