This adds a GC bitmap test of a type with many pointer bits and a
large scalar tail, such as the one in issue #11286. This test would
have failed prior to the fix in a8ae93fd. This test is a more direct
version of the test introduced in that commit (which was distilled
from the failing test in the issue).

Change-Id: I2e716cd1000b49bde237f5da6d857e8983fe7e7a
Reviewed-on: https://go-review.googlesource.com/11423Reviewed-by: Russ Cox <rsc@golang.org>