runtests: allow %EMPTY in <stdout> to verify no output

docs/tests/FILEFORMAT.md

@@ -755,6 +755,9 @@ a header and what is not in order to apply the CRLF line endings appropriately.
 
 `loadfile="filename"` makes loading the data from an external file.
 
+To verify that there was nothing sent to stdout, put `%EMPTY` as the only
+content.
+
 ### `<limit>`
 
 When this test runs and curl was built with debug enabled, runtests make sure

tests/runtests.pl

@@ -1273,6 +1273,8 @@ sub singletest_check {
     }
 
     if(@validstdout) {
+        $validstdout[0] =~ s/^%EMPTY[\r\n]*//;
+
         # verify redirected stdout
         my @actual = loadarray(stdoutfilename($logdir, $testnum));