net/http: test that ParseMultipartForm returns an error for int overflow
net/http: test that ParseMultipartForm returns an error for int overflow

ParseMultipartForm has been changed to return an error if maxMemory
parameter + 10MB causes int overflows. This adds a test for the new
behaviour.

For #40430

Change-Id: I4f66ce8a9382940182011d22a84ee52b1d1364cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/254977
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Trust: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go