TestStacking failures when default format is 2a

[Martin Pool]

With the default format set to 2a (per bug 398668) some stacking tests fail like so:

FAIL: per_branch.test_stacking.TestStacking.test_clone_stacking_policy_handling(BzrBranchFormat5)
    UnstackableBranchFormat not raised

FAIL: per_branch.test_stacking.TestStacking.test_clone_stacking_policy_handling(BzrBranchFormat6)
    UnstackableBranchFormat not raised

ERROR: per_branch.test_stacking.TestStacking.test_sprout_stacking_policy_handling(BzrBranchFormat5)
    The branch 'file:///tmp/testbzr-CUXNDJ.tmp/bzrlib.tests.per_branch.test_stacking.TestStacking.test_sprout_stacking_policy_handling%28BzrBranchFormat5%29/work/target/'(Branch format 5) is not a stackable format. You will need to upgrade the branch to permit branch stacking.

ERROR: per_branch.test_stacking.TestStacking.test_sprout_stacking_policy_handling(BzrBranchFormat6)
    The branch 'file:///tmp/testbzr-CUXNDJ.tmp/bzrlib.tests.per_branch.test_stacking.TestStacking.test_sprout_stacking_policy_handling%28BzrBranchFormat6%29/work/target/'(Branch format 6) is not a stackable format. You will need to upgrade the branch to permit branch stacking.

ERROR: per_branch.test_stacking.TestStacking.test_sprout_to_smart_server_stacking_policy_handling(BzrBranchFormat5)
    The branch 'bzr://127.0.0.1:54204/extra/target/'(Remote BZR Branch) is not a stackable format. You will need to upgrade the branch to permit branch stacking.

ERROR: per_branch.test_stacking.TestStacking.test_sprout_to_smart_server_stacking_policy_handling(BzrBranchFormat6)
    The branch 'bzr://127.0.0.1:52552/extra/target/'(Remote BZR Branch) is not a stackable format. You will need to upgrade the branch to permit branch stacking.

[Martin Pool (mbp)]

Presumably fixed by Robert's overall 2a branch.