varnishtest: Turn vtc_maxdur into a vtim_dur

We already pass it to VEV and VCLI subsystems in places where a
double is expected.

Trivia: we currently parse it in two distinct ways. So for now I'm
not eager to support duration units.

bin/varnishtest/vtc.h

@@ -76,7 +76,7 @@ extern volatile sig_atomic_t vtc_error; /* Error, bail out */
 extern int vtc_stop;		/* Abandon current test, no error */
 extern pthread_t	vtc_thread;
 extern int iflg;
-extern unsigned vtc_maxdur;
+extern vtim_dur vtc_maxdur;
 extern char *vmod_path;
 extern struct vsb *params_vsb;
 extern int leave_temp;

bin/varnishtest/vtc_main.c

@@ -94,7 +94,7 @@ struct vtc_job {
 
 
 int iflg = 0;
-unsigned vtc_maxdur = 60;
+vtim_dur vtc_maxdur = 60;
 static unsigned vtc_bufsiz = 1024 * 1024;
 
 static VTAILQ_HEAD(, vtc_tst) tst_head = VTAILQ_HEAD_INITIALIZER(tst_head);