Timing self test: increased duration

Increase the duration of the self test, otherwise it tends to fail on
a busy machine even with the recently upped tolerance. But run the
loop only once, it's enough for a simple smoke test.
diff --git a/ChangeLog b/ChangeLog
index 9fd8cce..c6031b4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -16,6 +16,8 @@
    * Allow comments in test data files.
    * The selftest program can execute a subset of the tests based on command
      line arguments.
+   * Improve the timing self-tests to be more robust when run on a
+     heavily-loaded machine.
 
 Bugfix
    * Fix ssl_parse_record_header() to silently discard invalid DTLS records