test-manifest: create constant for empty manifest
For symmetry with manifest v2, once we add that, let's extract a
constant for the empty v1 manifest.
'a' in d: True
d['a']: va
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'a' in d: False
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'e' in d: True
d['e']: ve
'b' in d: True
d['b']: vb2
'c' in d: True
d['c']: vc2
'd' in d: True
d['d']: vd
'e' in d: False
'f' in d: True
d['f']: vf
'b' in d: False
'c' in d: False
'd' in d: False
'e' in d: False
'f' in d: False