webdav: relax test to check for any redirect status, not just 301

CL 720820 changed net/http to use a 307 Temporary Redirect instead of
a 301 Moved Permanently when performing an automatic redirect under
some circumstances. Update tests in the webdav package to be agnostic
on the exact redirect status code.

Change-Id: I71784a738d18928a98387ddbd5475d50b19d15bf
Reviewed-on: https://go-review.googlesource.com/c/net/+/723120
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Auto-Submit: Nicholas Husin <nsh@golang.org>