build: add ability to disable some plugins from packaging and tests

When custom-packaging the VPP artifacts, it can be useful to exclude
some of the core plugins from packaging/testing, for some reasons.
A removal of a plugin(s) from the worktree needs to be tracked as
a separate change, and thus is tricky from the maintenance
point of view.

This change adds the ability to "pretend they do not exist" -
plugins which are added to the comma-separated environment
variable "VPP_EXCLUDED_PLUGINS" will not be added to the build
process and not packaged.

The tests do not have the 1:1 relationship as plugins,
so they might need to be modified separately. This change
includes some of these modifications as an example.

Type: feature
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: Id31562d00a01ced1acbb4996a633517cbd6f09d8

test/test_wireguard.py

@@ -6,6 +6,7 @@ import base64
 import os
 
 from hashlib import blake2s
+from config import config
 from scapy.packet import Packet
 from scapy.packet import Raw
 from scapy.layers.l2 import Ether, ARP
@@ -509,6 +510,9 @@ def is_handshake_init(p):
     return wg_p[Wireguard].message_type == 1
 
 
+@unittest.skipIf(
+    "wireguard" in config.excluded_plugins, "Exclude Wireguard plugin tests"
+)
 class TestWg(VppTestCase):
     """Wireguard Test Case"""
 
@@ -2848,6 +2852,9 @@ class WireguardHandoffTests(TestWg):
         """Multi-tunnel on the same port"""
 
 
+@unittest.skipIf(
+    "wireguard" in config.excluded_plugins, "Exclude Wireguard plugin tests"
+)
 class TestWgFIB(VppTestCase):
     """Wireguard FIB Test Case"""