fuzz: improve fuzz performance for accesslog and route (envoyproxy#8397)

This PR includes some performance improvements to route_fuzz_test.

gMock objects that are unmodified are now static (see recommendation by libFuzzer: https://llvm.org/docs/LibFuzzer.html#startup-initialization)
Exec/sec on my local machine with libFuzzer:

route_fuzz_test: 17 exec/sec -> 850 exec/sec
Risk level: Low

Signed-off-by: Asra Ali <[email protected]>

Author: asraa

test/common/router/route_fuzz_test.cc

@@ -36,7 +36,6 @@ cleanRouteConfig(envoy::api::v2::RouteConfiguration route_config) {
   // headers.
   envoy::api::v2::RouteConfiguration clean_config =
       replaceInvalidHeaders<envoy::api::v2::RouteConfiguration>(route_config);
-  // Replace invalid characters from
   auto internal_only_headers = clean_config.mutable_internal_only_headers();
   std::for_each(internal_only_headers->begin(), internal_only_headers->end(),
                 [](std::string& n) { n = Fuzz::replaceInvalidCharacters(n); });
@@ -69,9 +68,9 @@ cleanRouteConfig(envoy::api::v2::RouteConfiguration route_config) {
 
 // TODO(htuch): figure out how to generate via a genrule from config_impl_test the full corpus.
 DEFINE_PROTO_FUZZER(const test::common::router::RouteTestCase& input) {
+  static NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
+  static NiceMock<Server::Configuration::MockFactoryContext> factory_context;
   try {
-    NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
-    NiceMock<Server::Configuration::MockFactoryContext> factory_context;
     TestUtility::validate(input.config());
     ConfigImpl config(cleanRouteConfig(input.config()), factory_context, true);
     Http::TestHeaderMapImpl headers = Fuzz::fromHeaders(input.headers());