expr unit tests: compare length of each returned metric
expr unit tests: compare length of each returned metric

in 455cfcae6447e015bacab550a54a566ad54322d7
we accidentally remove the ability to compare if the len(actual.Values)
was the same as the len(want.Values)

this commit restores that feature

lyft/carbonapi,nnuss/carbonapi,errx/carbonapi,dgryski/carbonapi,errx/carbonapi,arodland/carbonapi