tests: net: coap_server: Adjust test filters

Add netif dependency for the test suite, as in other networking tests
and adjust minimal RAM and FLASH values.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

tests/net/lib/coap_server/common/testcase.yaml

@@ -1,5 +1,7 @@
 common:
-  min_ram: 16
+  min_ram: 40
+  min_flash: 180
+  depends_on: netif
   tags:
     - net
     - coap