Add log message injection to tests

By injecting messages into the maxscale.log from the test, the reader of
the log can easily see the "synchronization" with the test case. This does
affect the test timing so it can only be used to see whether non-timing
related functionality is correct.

maxscale-system-test/testconnections.cpp

@@ -1885,6 +1885,22 @@ void TestConnections::tprintf(const char* format, ...)
     fflush(stderr);
 }
 
+void TestConnections::log_printf(const char* format, ...)
+{
+    va_list argp;
+    va_start(argp, format);
+    int n = vsnprintf(nullptr, 0, format, argp);
+    va_end(argp);
+
+    va_start(argp, format);
+    char buf[n + 1];
+    vsnprintf(buf, sizeof(buf), format, argp);
+    va_end(argp);
+
+    maxscales->ssh_node_f(0, true, "echo '--- %s ---' >> /var/log/maxscale/maxscale.log", buf);
+    tprintf("%s", buf);
+}
+
 int TestConnections::get_master_server_id(int m)
 {
     int master_id = -1;