[regexp-builtins-fuzzer] Print generated source in verbose mode

My standard procedure for debugging regexp builtin fuzzer finds is to
turn on verbose mode and run the repro. This extends verbose output to
include the generated script which contains e.g. the regexp pattern,
the subject string, and the actual function call.

Tbr: yangguo@chromium.org
Bug: v8:8968
Change-Id: I0c7e930f4cbd34014f2781ca280919c5b002b049
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1511276Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60120}

test/fuzzer/regexp-builtins.cc

@@ -316,7 +316,13 @@ std::string GenerateSourceString(FuzzerArgs* args, const std::string& test) {
      << "const slow = test();\n"
      << "%SetForceSlowPath(false);\n";
   // clang-format on
-  return ss.str();
+
+  std::string source = ss.str();
+  if (kVerbose) {
+    fprintf(stderr, "Generated source:\n```\n%s\n```\n", source.c_str());
+  }
+
+  return source;
 }
 
 void PrintExceptionMessage(v8::Isolate* isolate, v8::TryCatch* try_catch) {