fuzz: use nanoseconds for SystemTime in RequestInfo. (envoyproxy#4255)

Avoids integer overflow that was previously occurring due to time in micros being too large, allows
exercise of nanosecond resolution formatters beyond all zero.

Fixes oss-fuzz issue https://oss-fuzz.com/v2/testcase-detail/5630125928873984.

Risk level: Low
Testing: Corpus entry added.

Signed-off-by: Harvey Tuch <htuch@google.com>

test/common/router/header_parser_corpus/clusterfuzz-testcase-minimized-header_parser_fuzz_test-5630125928873984

@@ -0,0 +1 @@
+headers_to_add {   header {     key: " "     value: "%START_TIME(�)%"   } } request_info {   start_time: 72059116831228591 }

test/fuzz/utility.h

@@ -36,7 +36,13 @@ inline test::fuzz::Headers toHeaders(const Http::HeaderMap& headers) {
 inline TestRequestInfo fromRequestInfo(const test::fuzz::RequestInfo& request_info) {
   TestRequestInfo test_request_info;
   test_request_info.metadata_ = request_info.dynamic_metadata();
-  test_request_info.start_time_ = SystemTime(std::chrono::microseconds(request_info.start_time()));
+#ifdef __APPLE__
+  // Clocks don't track at nanosecond on OS X.
+  test_request_info.start_time_ =
+      SystemTime(std::chrono::microseconds(request_info.start_time() / 1000));
+#else
+  test_request_info.start_time_ = SystemTime(std::chrono::nanoseconds(request_info.start_time()));
+#endif
   if (request_info.has_response_code()) {
     test_request_info.response_code_ = request_info.response_code().value();
   }