test: more robust detection of GOEXPERIMENT=unified
`go env GOEXPERIMENT` prints what experiments are enabled relative to
the baseline configuration, so it's not a very robust way to detect
what experiments have been statically enabled at bootstrap time.
Instead, we can check build.Default.ToolTags, which has goexperiment.*
for all currently enabled experiments, independent of baseline.
Change-Id: I6132deaa73b1e79ac24176ef4de5af67a507ee26
Reviewed-on: https://go-review.googlesource.com/c/go/+/422234
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: David Chase <drchase@google.com>