internal/fuzztest: factor out common fuzzer tests

All the fuzzers have the same test, which runs the fuzzer against every
entry in the corpus. Move the test logic into a separate package.

Change-Id: I3a7e2ca75d20a5ff6d51ed9e6151629e6667684b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/212258
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/fuzztest/fuzztest.go b/internal/fuzztest/fuzztest.go
new file mode 100644
index 0000000..9c066dc
--- /dev/null
+++ b/internal/fuzztest/fuzztest.go
@@ -0,0 +1,44 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fuzztest contains a common fuzzer test.
+package fuzztest
+
+import (
+	"flag"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sort"
+	"testing"
+)
+
+var corpus = flag.String("corpus", "corpus", "directory containing the fuzzer corpus")
+
+// Test executes a fuzz function for every entry in the corpus.
+func Test(t *testing.T, fuzz func(b []byte) int) {
+	dir, err := os.Open(*corpus)
+	if err != nil {
+		t.Fatal(err)
+	}
+	infos, err := dir.Readdir(0)
+	if err != nil {
+		t.Fatal(err)
+
+	}
+	var names []string
+	for _, info := range infos {
+		names = append(names, info.Name())
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		t.Run(name, func(t *testing.T) {
+			b, err := ioutil.ReadFile(filepath.Join(*corpus, name))
+			if err != nil {
+				t.Fatal(err)
+			}
+			fuzz(b)
+		})
+	}
+}