net/http: add tests covering non-GET methods for file serving

net/http: add tests covering non-GET methods for file serving

ServeFile and FileServer will respond to methods such as DELETE by
serving the file contents. This is surprising, but we don't want to
change it without some consideration.

Add tests covering the current behavior.

For #59470

Change-Id: Ib6a2594c5b2b7f380149fc1628f7204b308161e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/482876
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>