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 d69f5c5..d7101c0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,7 +6,8 @@
    * Allow comments in test data files.
    * The selftest program can execute a subset of the tests based on command
      line arguments.
-   * New unit tests for timing.
+   * New unit tests for timing. Improve the self-test to be more robust
+     when run on a heavily-loaded machine.
 
 Bugfix
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.