test suite: saver check if bzr is installed

Test if bzrlib.__doc__ is not empty. Ignores an empty lib.

tests/hghave

@@ -27,7 +27,7 b' def has_baz():'
 def has_bzr():
     try:
         import bzrlib
-        return True
+        return bzrlib.__doc__ != None
     except ImportError:
         return False