Use /tmp as TMPDIR if not otherwise set.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4573 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishtest/vtc.c

@@ -643,6 +643,7 @@ main(int argc, char * const *argv)
 	init_macro();
 	init_sema();
 
+	setenv("TMPDIR", "/tmp", 0);
 	vtc_tmpdir = tempnam(NULL, "vtc");
 	AN(vtc_tmpdir);
 	AZ(mkdir(vtc_tmpdir, 0700));