selftest: allow running a subset of the tests

If given command line arguments, interpret them as test names and only
run those tests.
diff --git a/ChangeLog b/ChangeLog
index 2061be0..80e44dd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,6 +4,8 @@
 
 Features
    * Allow comments in test data files.
+   * The selftest program can execute a subset of the tests based on command
+     line arguments.
 
 Bugfix
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.