Initial triage: Add sickbay.yaml for eos_multicast_base lab (#73)

## Summary
Initial triage of the eos_multicast_base lab that was never tested
before. This PR adds proper expected failure tracking for validation
discrepancies in a multicast routing environment.

## Changes
- Created GitHub issue #72 documenting multicast routing validation
failures in the eos_multicast_base lab
- Added `sickbay.yaml` file marking 3 expected failures as `xfailed`
with proper GitHub issue references
- Lab now runs cleanly with 23 passed tests and 3 expected failures
(xfailed)

## Lab Details
- **Topology**: PIM SSM multicast routing with 3 Arista EOS devices
(rp1, rp2, rcvr) and 2 IOS test harness devices
- **Technology**: PIM SSM operation with static RPs, IGMP version 3,
OSPF as underlying unicast routing
- **Failures**: Main RIB route validation discrepancies affecting OSPF
route classification and metrics

## Test Results
Before: 3 failed, 23 passed
After: 23 passed, 3 xfailed

The lab is now properly integrated into the test suite with documented
expected failures for future investigation and resolution.

## Next Steps
The created GitHub issue #72 outlines investigation areas for Batfish
multicast routing modeling improvements, particularly around OSPF route
classification consistency and PIM SSM interaction effects.

Author: dhalperi

snapshots/eos_multicast_base/validation/sickbay.yaml

@@ -0,0 +1,13 @@
+entries:
+  - hostname: rp1
+    test_name: test_main_rib_routes
+    skip:
+      reason: https://github.com/batfish/lab-validation/issues/72
+  - hostname: rp2
+    test_name: test_main_rib_routes
+    skip:
+      reason: https://github.com/batfish/lab-validation/issues/72
+  - hostname: rcvr
+    test_name: test_main_rib_routes
+    skip:
+      reason: https://github.com/batfish/lab-validation/issues/72