Try fixing the u00001 instability by retrying polls which fail

with EINTR.

I suspect the underlying issue is that SIGCHLD is being sent
to a random thread and that happens to be the s1 thread on some
platforms.

bin/varnishtest/vtc_http.c

@@ -387,6 +387,8 @@ http_rxchar(struct http *hp, int n, int eof)
 		pfd[0].events = POLLIN;
 		pfd[0].revents = 0;
 		i = poll(pfd, 1, hp->timeout);
+		if (i < 0 && errno == EINTR)
+			continue;
 		if (i == 0)
 			vtc_log(hp->vl, hp->fatal,
 			    "HTTP rx timeout (fd:%d %u ms)",
@@ -1215,6 +1217,8 @@ cmd_http_expect_close(CMD_ARGS)
 		fds[0].events = POLLIN | POLLERR;
 		fds[0].revents = 0;
 		i = poll(fds, 1, hp->timeout);
+		if (i < 0 && errno == EINTR)
+			continue;
 		if (i == 0)
 			vtc_log(vl, hp->fatal, "Expected close: timeout");
 		if (i != 1 || !(fds[0].revents & (POLLIN|POLLERR)))