bgpv1: Avoid binding to ephemeral ports in tests

[rastislavs]

The tests in:

• pkg/bgpv1/test ("BGP component tests")
• pkg/bgpv1/manager/reconcile_test.go ("Reconciler unit tests")

bind to ports above 1023 (in particular: 1790-1794). This can cause issues on some systems where these ports fall under the ephemeral port number range. Although it should be fine in most Linux distributions which usually default the ephemeral port range to 32768-60999, we should avoid this.

For the "BGP component tests":

• We can use the IANA-assigned port for BGP (179) for all BGP instances in the test, given that they do not bind to ::179 / 0.0.0.0:179, but use distinct bind address (Global.ListenAddresses in GoBGP). That may require adding a new way of exposing this config for the Cilium agent's BGP instance, for example with a new annotation similar to the existing local-port one.

For the "Reconciler unit tests":

• These tests can mock GoBGP so that binding is not really needed anymore,
• or we can make the necessary tests privileged and used IANA-assigned port numbers - however, apart from the BGP one (179), we would need one more as the test exercises the change of the port.

[github-actions]

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