sidedata: use revlogv2 requirement in the test helper

We are about to drop the requirement dedicated to sidedata (since revlogv2 will
support them unconditionally and previous version will not.).

To prepare this more we adapt the test code to not use that requirements.

Differential Revision: https://phab.mercurial-scm.org/D10615

tests/testlib/ext-sidedata.py

@@ -60,7 +60,7 def wrapget_sidedata_helpers(orig, srcre
     assert not computers and not removers  # deal with composition later
     addedreqs = dstrepo.requirements - srcrepo.requirements
 
-    if requirements.SIDEDATA_REQUIREMENT in addedreqs:
+    if requirements.REVLOGV2_REQUIREMENT in addedreqs:
 
         def computer(repo, revlog, rev, old_sidedata):
             assert not old_sidedata  # not supported yet