give timing critical test a bit more margin

we want to make sure that we do not hit timeout_idle as long as we stay
below it, but the margin of just 100ms has triggered a false negative.
Double the margin to 200ms...

see with vtest (c4u lines extracted)

**** c4u  Loop #3
**   c4u  === delay 0.3
***  c4u  delaying 0.3 second(s)
**** dT   2.025
**   c4u  === sendhex 0d0a
**** c4u  sendhex|  0d 0a
---- c4u  Write failed: Socket is not connected

bin/varnishtest/tests/b00067.vtc

@@ -82,10 +82,10 @@ client c2u -connect "${tmpdir}/v1.sock" {
 client c3u -connect "${tmpdir}/v1.sock" {
 	loop 3 {
 		# send a periodic CRLF
-		delay 0.3
+		delay 0.2
 		sendhex 0d0a
 	}
-	delay 0.3
+	delay 0.4
 	expect_close
 } -start
 
@@ -94,10 +94,10 @@ client c4u -connect "${tmpdir}/v1.sock" {
 	rxresp
 	loop 3 {
 		# send a periodic CRLF
-		delay 0.3
+		delay 0.2
 		sendhex 0d0a
 	}
-	delay 0.3
+	delay 0.4
 	expect_close
 } -start