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.