tests: net: ipv4_fragment: Add PMTU testing

Enable PMTU so that we test it with IPv4 fragmentation code.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

tests/net/ipv4_fragment/testcase.yaml

@@ -5,9 +5,14 @@
 #
 common:
   depends_on: netif
+  tags:
+    - net
+    - ipv4
+    - fragment
 tests:
   net.ipv4.fragment:
-    tags:
+    extra_configs:
-      - net
+      - CONFIG_NET_IPV4_PMTU=n
-      - ipv4
+  net.ipv4.fragment.with_pmtu:
-      - fragment
+    extra_configs:
+      - CONFIG_NET_IPV4_PMTU=y