cmd/go: add tracing instrumentation to load.TestPackagesFor

This change adds tracing instrumentation into load.TestPackagesFor,
propagating context through its callers.

Updates #38714

Change-Id: I80fefaf3116ccccffaa8bb7613a656bda867394c
Reviewed-on: https://go-review.googlesource.com/c/go/+/248323


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>