Add l2xc support, including extensive tests to validate correct usage of the feature

unittest/correct-l2xc.yaml

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+test:
+  description: "A few correct ways of setting L2 cross connects"
+  errors:
+    count: 0
+---
+bondethernets:
+  BondEthernet0:
+    interfaces: [ GigabitEthernet3/0/0, GigabitEthernet3/0/1 ]
+
+interfaces:
+  GigabitEthernet1/0/0:
+    description: "Cross connected to Gi1/0/1"
+    l2xc: GigabitEthernet1/0/1
+  GigabitEthernet1/0/1:
+    description: "Cross connected to Gi1/0/0"
+    l2xc: GigabitEthernet1/0/0
+
+  GigabitEthernet2/0/0:
+    description: "Cross connected to Gi2/0/1.100"
+    l2xc: GigabitEthernet2/0/1.100
+  GigabitEthernet2/0/1:
+    description: "Main phy with a subint"
+    sub-interfaces:
+      100:
+        description: "Cross connected to Gi2/0/0"
+        l2xc: GigabitEthernet2/0/0
+
+  GigabitEthernet3/0/0:
+    mtu: 3000
+  GigabitEthernet3/0/1:
+    mtu: 3000
+
+  BondEthernet0:
+    description: "BE0 with two xconnected sub-ints"
+    sub-interfaces:
+      100:
+        description: "Cross connected to BE0.101"
+        l2xc: BondEthernet0.101
+      101:
+        description: "Cross connected to BE0.100"
+        l2xc: BondEthernet0.100

unittest/error-bridgedomain7.yaml

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+test:
+  description: "An interface that is in a bridgedomain, cannot also be an l2 cross connect"
+  errors:
+    expected:
+     - "interface .* l2xc target .* is in a bridgedomain"
+    count: 1
+---
+interfaces:
+  GigabitEthernet1/0/0:
+    mtu: 3000
+  GigabitEthernet1/0/1:
+    mtu: 3000
+
+  GigabitEthernet1/0/1:
+    mtu: 3000
+    l2xc: GigabitEthernet1/0/0
+
+bridgedomains:
+  bd10:
+    description: "Bridge Domain 10 has Gi1/0/0 which is also a target of an L2XC"
+    mtu: 3000
+    interfaces: [ GigabitEthernet1/0/0, GigabitEthernet1/0/1 ]

unittest/error-l2xc1.yaml

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+test:
+  description: "L2 cross connect targets cannot occur more than once"
+  errors:
+    expected:
+     - "interface .* l2xc target .* is not unique"
+    count: 2
+---
+interfaces:
+  GigabitEthernet1/0/0:
+    description: "Cross connected to Gi1/0/1"
+    l2xc: GigabitEthernet1/0/1
+  GigabitEthernet1/0/1:
+    description: "Cross connected to Gi1/0/0"
+    l2xc: GigabitEthernet1/0/0
+
+  GigabitEthernet2/0/0:
+    description: "Cross connected to Gi1/0/0 as well"
+    l2xc: GigabitEthernet1/0/0

unittest/error-l2xc2.yaml

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+test:
+  description: "L2 cross connect targets must exist"
+  errors:
+    expected:
+     - "interface .* l2xc target .* does not exist"
+    count: 1
+---
+interfaces:
+  GigabitEthernet1/0/0:
+    description: "Cross connected to Gi1/0/1"
+    l2xc: GigabitEthernet1/0/1
+  GigabitEthernet1/0/1:
+    description: "Cross connected to Gi1/0/2, which does not exist"
+    l2xc: GigabitEthernet1/0/2

unittest/error-l2xc3.yaml

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+test:
+  description: "L2 cross connect targets cannot also occur in a bridgedomain"
+  errors:
+    expected:
+     - "interface .* l2xc target .* in a bridgedomain"
+    count: 1
+---
+interfaces:
+  GigabitEthernet1/0/0:
+    description: "Cross connected to Gi1/0/1"
+    l2xc: GigabitEthernet1/0/1
+  GigabitEthernet1/0/1:
+    description: "In a Bridge Domain, so cannot be a target of L2XC"
+
+bridgedomains:
+  bd10:
+    description: "A Bridge with gi1/0/0 which also occurs as an L2XC target"
+    interfaces: [ GigabitEthernet1/0/1 ]

unittest/error-l2xc4.yaml

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+test:
+  description: "L2 cross connect targets cannot have an IP address or LCP"
+  errors:
+    expected:
+     - "interface .* l2xc so it cannot be an LCP"
+     - "interface .* l2xc so it cannot have an address"
+    count: 3
+---
+interfaces:
+  GigabitEthernet1/0/0:
+    description: "Cross connected to Gi1/0/1, but should not have an LCP"
+    l2xc: GigabitEthernet1/0/1
+    lcp: "e1-0-0"
+  GigabitEthernet1/0/1:
+    description: "Cross connected to Gi1/0/0, but should not have address and LCP"
+    l2xc: GigabitEthernet1/0/0
+    lcp: "e1-0-1"
+    addresses: [ 192.0.2.1/30 ]

unittest/error-l2xc5.yaml

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+test:
+  description: "L2 cross connect from a phy cannot also have sub-interfaces"
+  errors:
+    expected:
+     - "interface .* l2xc so it cannot have sub-interfaces"
+    count: 1
+---
+interfaces:
+  GigabitEthernet1/0/0:
+    l2xc: GigabitEthernet1/0/1.100
+    sub-interfaces:
+      100:
+        description: "If the parent is cross connected, it should not have sub-interfaces"
+
+  GigabitEthernet1/0/1:
+    sub-interfaces:
+      100:
+        l2xc: GigabitEthernet1/0/0