go/internal/srcimporter: set -mod=vendor before running tests

go/internal/srcimporter: set -mod=vendor before running tests

Otherwise, if the working directory is inside a standard-library
module, the test may try to fetch module contents from GOPROXY or
upstream.

Updates #26924
Updates #30228
Updates #30241

Change-Id: I4cb9a07721bd808fd094f7ed55a74cf7bce9cd6f
Reviewed-on: https://go-review.googlesource.com/c/164625
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>