weak: fix weak pointer test to correctly iterate over weak pointers after GC

weak: fix weak pointer test to correctly iterate over weak pointers after GC

This change fixes a bug in the weak pointer test where the loop was
attempting to iterate over a nil slice (bt) instead of the weak
pointer slice (wt).

After setting bt to nil and running GC, the test should iterate over
the weak pointers to verify they've been cleared, not attempt to
iterate over the now-nil strong references.

Change-Id: Ic0425f59da132257770ed87d1bcea5d2c0a54e07
Reviewed-on: https://go-review.googlesource.com/c/go/+/723600
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>