net/url: reject query values with semicolons

Semicolons are no longer valid separators, so
net/url.ParseQuery will now return an error
if any part of the query contains a semicolon.

net/http.(*Request).ParseMultipartForm has been
changed to fall through and continue parsing
even if the call to (*Request).ParseForm fails.

This change also includes a few minor refactors
to existing tests.

Fixes #25192

Change-Id: Iba3f108950fb99b9288e402c41fe71ca3a2ababd
Reviewed-on: https://go-review.googlesource.com/c/go/+/325697


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>