test: add a field tracking test

Now that we can set experiments at build time instead of make.bash time,
we can actually write a test for field tracking!

Update #20014

This CL contains a test for the functionality fixed in CL 312069.

Change-Id: I7569a7057bbc7c88ae25ae7bf974b0c8a4e35be8
Reviewed-on: https://go-review.googlesource.com/c/go/+/312217


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>