Add fuzz test for FormatDSN

Run (go 1.18+): go test -fuzz FuzzFormatDSN

Note: invalid host:addr values are currently ignored as they are known
to break (ParseDSN doesn't strictly check address format).

dsn_fuzz_test.go

@@ -0,0 +1,47 @@
+//go:build go1.18
+// +build go1.18
+
+package mysql
+
+import (
+	"net"
+	"testing"
+)
+
+func FuzzFormatDSN(f *testing.F) {
+	for _, test := range testDSNs { // See dsn_test.go
+		f.Add(test.in)
+	}
+
+	f.Fuzz(func(t *testing.T, dsn1 string) {
+		// Do not waste resources
+		if len(dsn1) > 1000 {
+			t.Skip("ignore: too long")
+		}
+
+		cfg1, err := ParseDSN(dsn1)
+		if err != nil {
+			t.Skipf("invalid DSN: %v", err)
+		}
+
+		dsn2 := cfg1.FormatDSN()
+		if dsn2 == dsn1 {
+			return
+		}
+
+		// Skip known cases of bad config that are not strictly checked by ParseDSN
+		if _, _, err := net.SplitHostPort(cfg1.Addr); err != nil {
+			t.Skipf("invalid addr %q: %v", cfg1.Addr, err)
+		}
+
+		cfg2, err := ParseDSN(dsn2)
+		if err != nil {
+			t.Fatalf("%q rewritten as %q: %v", dsn1, dsn2, err)
+		}
+
+		dsn3 := cfg2.FormatDSN()
+		if dsn3 != dsn2 {
+			t.Errorf("%q rewritten as %q", dsn2, dsn3)
+		}
+	})
+}