ENH: Add keepdims to linalg.norm
MAINT: Stop repeatedly running duplicate tests.
    These three tests don't make use of the infrastructre in
    _TestNorm. This means they are identical tests when run
    as part of TestNorm(Double,Single,Int64). Once is enough.
Merge pull request #5196 from ewmoore/norm_keepdims

Add keepdims to linalg.norm