JSSpecCompiler: Add option to silence all diagnostics and use in tests

In the future, we will probably want to test regressions in diagnostics
too but let's not do that for now.

Tests/JSSpecCompiler/test-runner.cpp

@@ -92,6 +92,8 @@ Vector<ByteString> build_command_line_arguments(LexicalPath const& test_source,
     if (test_source.has_extension(".cpp"sv))
         result.append("-xc++"sv);
 
+    result.append("--silence-diagnostics"sv);
+
     result.append(test_source.string());
 
     return result;