net/url: modernise parse and unit tests

net/url: modernise parse and unit tests

Remove the naked returns and goto statements from parse.

Make tests more consistent in the got/want ordering, and clean up some
unnecessary helper functions.

Change-Id: Iaa244cb8c00dd6b42836d95448bf02caa72bfabd
Reviewed-on: https://go-review.googlesource.com/28890
Reviewed-by: Ian Lance Taylor <iant@golang.org>