added testing instructions

Author: jerryzhou196

flow/api/parse/pdf/pdftotext.cc

@@ -16,6 +16,7 @@ using ByteArray = poppler::byte_array;
 using Document = poppler::document;
 using Page = poppler::page;
 
+// test this file with `make test`
 namespace {
     OnceFlag errorFnFlag;