internal/fuzz: minimization should not reduce coverage

Minimization should result in a fuzz input which
includes the same coverage bits as the original
input.

Updates #48326

Change-Id: I6c5f30058b57ccd1a096ad0e9452a4dfbb7d9aab
Reviewed-on: https://go-review.googlesource.com/c/go/+/391454


Trust: Bryan Mills <bcmills@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>