fuzz: use nanoseconds for SystemTime in RequestInfo.

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 <[email protected]>

Author: htuch

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 +36,7 @@ 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()));
+  test_request_info.start_time_ = SystemTime(std::chrono::nanoseconds(request_info.start_time()));
   if (request_info.has_response_code()) {
     test_request_info.response_code_ = request_info.response_code().value();
   }