[dev.fuzz] cmd/go: fix test_fuzz_cache

This test started failing when coverage-based fuzzing was enabled.
It expects at least one file to be written to the fuzz cache. Nothing
was written because the fuzz function was trivial, and no interesting
inputs could be discovered.

This CL makes the fuzz function return different values for different
inputs, which is enough to pass.

Change-Id: I6ffd2667891cf5f3e4588133efb65f096a739c09
Reviewed-on: https://go-review.googlesource.com/c/go/+/319871
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>

src/cmd/go/testdata/script/test_fuzz_cache.txt

@@ -39,7 +39,15 @@ import "testing"
 
 func FuzzY(f *testing.F) {
 	f.Add([]byte("y"))
-	f.Fuzz(func(t *testing.T, b []byte) {})
+	f.Fuzz(func(t *testing.T, b []byte) { Y(b) })
+}
+-- y.go --
+package y
+
+import "bytes"
+
+func Y(b []byte) bool {
+	return bytes.Equal(b, []byte("y"))
 }
 -- empty/empty.go --
 package empty