test: switch fieldtrack test to use GOEXPERIMENT

Now that we can set GOEXPERIMENT at build time, we no longer need
-d=fieldtrack in the compiler to enabled field tracking at build time.
Switch the one test that uses -d=fieldtrack to use GOEXPERIMENT
instead so we can eliminate this debug flag and centralize on
GOEXPERIMENT.

Updates #42681.

Change-Id: I14c352c9a97187b9c5ec8027ff672d685f22f543
Reviewed-on: https://go-review.googlesource.com/c/go/+/302969


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>