internal/fuzz: properly handle duplicates during cache loading

When loading the corpus, if the cache contained an entry which was a
duplicate of an entry added using f.Add, coordinator.addCorpusEntries
would return early, ignoring everything after this entry in the cache.

Instead, skip duplicates as intended, and continue to load the rest of
the cache.

Fixes #50913

Change-Id: I3a64b93cbb217c5c364a9f8d0005752e9e9d10ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/381960


Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>