Add simulation tests

Use clknetsim to run multiple chronyd instances with simulated clocks
and network. It allows fast and reproducible testing, without real
network.

Included are several tests of performance in different clock/network
conditions, chronyd options, NTP authentication, chronyc, and past bug
fixes.

test/simulation/005-externalstep

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+. test.common
+
+test_start "external time step"
+
+min_sync_time=1300
+max_sync_time=1500
+
+for step in -1e2 1e2; do
+	# Make one step in 150th second
+	client_step="(* $step (equal 0.1 (sum 1.0) 150))"
+	run_test || test_fail
+	check_chronyd_exit || test_fail
+	check_sync || test_fail
+done
+
+test_pass