Define a debug bit to mark that we are running under varnishtest control

and always set it in varnishtest

bin/varnishtest/vtc_varnish.c

@@ -390,6 +390,7 @@ varnish_launch(struct varnish *v)
 	VSB_printf(vsb, " -p syslog_cli_traffic=off");
 	VSB_printf(vsb, " -p sigsegv_handler=on");
 	VSB_printf(vsb, " -p thread_pool_min=10");
+	VSB_printf(vsb, " -p debug=+vtc_mode");
 	VSB_printf(vsb, " -a '%s'", "127.0.0.1:0");
 	VSB_printf(vsb, " -M '%s %s'", abuf, pbuf);
 	VSB_printf(vsb, " -P %s/varnishd.pid", v->workdir);

include/tbl/debug_bits.h

@@ -39,3 +39,4 @@ DEBUG_BIT(VCLREL,		vclrel,		"Rapid VCL release")
 DEBUG_BIT(LURKER,		lurker,		"VSL Ban lurker")
 DEBUG_BIT(ESI_CHOP,		esi_chop,	"Chop ESI fetch to bits")
 DEBUG_BIT(FLUSH_HEAD,		flush_head,	"Flush after http1 head")
+DEBUG_BIT(VTC_MODE,		vtc_mode,	"Varnishtest Mode")