tests/netgraph: Fix build error due to missing directory

While MFCing the tests into stable/11 the test directory needs to be
added, because earlier netgraph tests were not MFCed to this train.

PR: 256773
Reported by: Arrigo Marchiori
Submitted by: Arrigo Marchiori
Tested by: Arrigo Marchiori

This is a direct commit into the stable/12 branch.