tests/fuzzers: improve the fuzzers (#21829)

* tests/fuzzers, common/bitutil: make fuzzers use correct returnvalues + remove output

* tests/fuzzers/stacktrie: fix duplicate-key insertion in stacktrie (false positive)

* tests/fuzzers/stacktrie: fix compilation error

* tests/fuzzers: linter nits

tests/fuzzers/keystore/keystore-fuzzer.go

@@ -33,5 +33,5 @@ func Fuzz(input []byte) int {
 		panic(err)
 	}
 	os.Remove(a.URL.Path)
-	return 0
+	return 1
 }