Avoid runtests.sh log spew

Summary:
Not every test produces a .log file. So when a test fails and it
doesn't have a .log file, the test runner spews. This fixes.

Reviewed By: samwgoldman

Differential Revision: D4421443

fbshipit-source-id: 87fbe39e104f9627215e88eb072db671b7db86d6

Author: gabelevi

runtests.sh

@@ -322,7 +322,7 @@ runtest() {
         if [ -s "$OUT_DIR/$diff_file" ]
         then
             mv "$OUT_DIR/$out_file" "$dir"
-            mv "$OUT_DIR/$log_file" "$dir"
+            [ -s "$OUT_DIR/$log_file" ] && mv "$OUT_DIR/$log_file" "$dir"
             mv "$OUT_DIR/$err_file" "$dir"
             mv "$OUT_DIR/$diff_file" "$dir"
             return_status=$RUNTEST_FAILURE