cmd/go/internal/test: prepend -test.timeout rather than appending

cmd/go/internal/test: prepend -test.timeout rather than appending

Tests may accept positional arguments, in which case the -test.timeout
flag must be passed before those arguments.

Fixes #34072

Change-Id: I5b92d7c0edc4f9e1efb63b0733937b76236c0eff
Reviewed-on: https://go-review.googlesource.com/c/go/+/193297
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>