test: stp_basic: Stabilize test

With the new yangerd it failed 100% of the times, but it could fail
before as well, but that whould be very rare. Wait until operational
has been updated.

test/case/interfaces/bridge_stp_basic/test.py

@@ -167,6 +167,8 @@ with infamy.Test() as test:
                  lambda: explicit_prios(c), lambda: explicit_prios(d))
 
     with test.step("Verify that A, B, C and D agrees on A being the root bridge"):
+        # Snapshot A's bridge-id only once it reflects the configured priority (1); oper data lags config.
+        until(lambda: (bridge_id(a, "bridge-id") or "").startswith("1."), 120)
         a_id = bridge_id(a, "bridge-id")
         print(f"A's bridge-id: {a_id}")
         until(lambda: all(map(lambda n: bridge_id(n, "root-id") == a_id, (a, b, c, d))), 120)