internal/buildcfg: enable greenteagc experiment by default

internal/buildcfg: enable greenteagc experiment by default

Slightly bump the value in Test/wasmmemsize.go. We use 1 additional page
compared to before, and we were already sitting *right* on the edge.

For #73581.

Change-Id: I485df16c3cf59803a8a1fc852b3e90666981ab09
Reviewed-on: https://go-review.googlesource.com/c/go/+/656195
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>