message: test: fix expectation
  Redo of
diff: @@ -417,7 +417,7 @@ TEST_P(AdminInstanceTest, EscapeHelpTextWithPunctuation) {
  Buffer::OwnedImpl response;
  EXPECT_EQ(Http::Code::OK, getCallback("/", header_map, response));
  Http::HeaderString& content_type = header_map.ContentType()->value();
  - EXPECT_TRUE(content_type.find("text/html")) << content_type.c_str();
  + EXPECT_THAT(std::string(content_type.getStringView()), testing::HasSubstr("text/html"));
  EXPECT_EQ(-1, response.search(planets.data(), planets.size(), 0));
  const std::string escaped_planets = "jupiter&gt;saturn&gt;mars";
  EXPECT_NE(-1, response.search(escaped_planets.data(), escaped_planets.size(), 0));
  