[escher][test] Avoid taking address of vector<bool> element

This is not allowed in C++ and newer libc++ implementations generate an
error. Instead, capture a reference to the whole vector and offset and
assign directly to the element.

Change-Id: Ie5ce21603d0ad01f9b7e256dacae09d2dc327000
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1563473
Reviewed-by: Yilong Li <liyl@google.com>
Fuchsia-Auto-Submit: James Robinson <jamesr@google.com>
Commit-Queue: James Robinson <jamesr@google.com>
Reviewed-by: Petr Hosek <phosek@google.com>