net/http: deflake TestRequestBodyLimit

net/http: deflake TestRequestBodyLimit

This test can return with a Transport still processing
an in-flight request, resulting in a test failure due
to the leaked Transport.

Avoid this by waiting for the Transport to close the
request body before returning.

Fixes #60264

Change-Id: I8d8b54f633c2e28da2b1bf1bc01ce09dd77769de
Reviewed-on: https://go-review.googlesource.com/c/go/+/522695
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>