Fix flaky tests on Ubuntu 22.04

It is necessary to handle OpenSSL errors very carefully. Otherwise,
errors may appear in unexpected places. For example, we didn't catch
an error from EVP_DigestInit_ex() and it appears sometimes in conn.go:

func (c *Conn) getErrorHandler(rv C.int, errno error) func() error {
	errcode := C.SSL_get_error(c.ssl, rv) // <- here

md4.go

@@ -51,8 +51,11 @@ func (s *MD4Hash) Close() {
 }
 
 func (s *MD4Hash) Reset() error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
 	if C.X_EVP_DigestInit_ex(s.ctx, C.X_EVP_md4(), engineRef(s.engine)) != 1 {
-		return errors.New("openssl: md4: cannot init digest ctx")
+		return errors.New("openssl: md4: cannot init digest ctx: " +
+			errorFromErrorQueue().Error())
 	}
 	return nil
 }