Checking that rules that add additional attributes correctly add those attributes and
overwrite existing ones.

-- rules --
data is hello, world!
attr add b=NEW
attr add c=baz
plumb to edit

-- initial --
src: bash
dst: ad
wdir: /home/foo/bar
attrs: a=foo b=bar
ndata: 13
data: hello, world!
-- processed --
src: bash
dst: ad
wdir: /home/foo/bar
attrs: a=foo b=NEW c=baz
ndata: 13
data: hello, world!
