runtime: use Escape instead of escape in export_test.go

runtime: use Escape instead of escape in export_test.go

I landed the bottom CL of my stack without rebasing or retrying trybots,
but in the rebase "escape" was removed in favor of "Escape."

Change-Id: Icdc4d8de8b6ebc782215f2836cd191377cc211df
Reviewed-on: https://go-review.googlesource.com/c/go/+/403755
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>