cmd/internal/objabi,test: use correct GOEXPERIMENT build tags in test/run.go

Currently, run.go sets GOEXPERIMENT build tags based on the
*difference* from the baseline experiment configuration, rather than
the absolute experiment configuration. This differs from cmd/go. As a
result, if we set a baseline configuration and don't override it with
a GOEXPERIMENT setting, run.go won't set any GOEXPERIMENT build tags,
instead of setting the tags corresponding to the baseline
configuration.

Fix this by making compile -V=goexperiment produce the full
GOEXPERIMENT configuration, which run.go can then use to set exactly
the right set of build tags.

For #40724.

Change-Id: Ieda6ea62f1a1fabbe8d749d6d09c198fd5ca8377
Reviewed-on: https://go-review.googlesource.com/c/go/+/310171


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>