Disable flaky tests reported in #6343 / #6345 (#6346)

test: zombienet-polkadot-functional-0018-shared-core-idle-parachain
Disable flaky test reported in
https://github.com/paritytech/polkadot-sdk/issues/6343

test: zombienet-polkadot-functional-0016-approval-voting-parallel
Disable flaky test reported in
https://github.com/paritytech/polkadot-sdk/issues/6345

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

.gitlab/pipeline/zombienet/polkadot.yml

@@ -225,7 +225,7 @@ zombienet-polkadot-functional-0015-coretime-shared-core:
       --local-dir="${LOCAL_DIR}/functional"
       --test="0015-coretime-shared-core.zndsl"
 
-zombienet-polkadot-functional-0016-approval-voting-parallel:
+.zombienet-polkadot-functional-0016-approval-voting-parallel:
   extends:
     - .zombienet-polkadot-common
   script:
@@ -241,7 +241,7 @@ zombienet-polkadot-functional-0017-sync-backing:
       --local-dir="${LOCAL_DIR}/functional"
       --test="0017-sync-backing.zndsl"
 
-zombienet-polkadot-functional-0018-shared-core-idle-parachain:
+.zombienet-polkadot-functional-0018-shared-core-idle-parachain:
   extends:
     - .zombienet-polkadot-common
   before_script: