cmd/go: process -debug-trace flag for cmd/test and cmd/vet

These commands are build-like commands that do their own flag
processing, so the value of debug-trace isn't available until
the command starts running. Start tracing in the cmd's run
function.

Updates #38714

Change-Id: I4d633e6ee907bf09feac52c2aff3daceb9b20e12
Reviewed-on: https://go-review.googlesource.com/c/go/+/248324


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