[dev.typeparams] test: test regabidefers in live.go

Previously, live.go is conditioned on not using regabidefers. Now
we have regabidefers enabled by default everywhere, and we may
remove the fallback path in the near future, test that
configuration instead.

Change-Id: Idf910aee323bdd6478bc7a2062b2052d82ce003f
Reviewed-on: https://go-review.googlesource.com/c/go/+/325111


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>