Tests: always define temp paths.

This is required since nginx tries to create them at startup (and fails if
it can't).
diff --git a/ssi-include-big.t b/ssi-include-big.t
index d132794..6794b25 100644
--- a/ssi-include-big.t
+++ b/ssi-include-big.t
@@ -36,6 +36,10 @@
     access_log    off;
     root          %%TESTDIR%%;
 
+    client_body_temp_path  %%TESTDIR%%/client_body_temp;
+    fastcgi_temp_path      %%TESTDIR%%/fastcgi_temp;
+    proxy_temp_path        %%TESTDIR%%/proxy_temp;
+
     output_buffers  2 512;
     ssi on;
     gzip on;