system-tests: Set stable datapath-id on bridges in system tests.

When a port with a globally-administered MAC address (e.g.,
00:00:00:00:00:04) is added to br-int, OVS may recalculate the
bridge datapath ID if no explicit datapath-id is configured.
The new DPID causes OVS to tear down all OpenFlow connections
to the bridge, since there is no OpenFlow message to notify
the controller of a DPID change.

This triggers ovn-controller's ofctrl_seqno_flush(), which
resets the internal sequence counter to 0. If a flow update
from before the reconnection is still in-flight, ofctrl_put()
detects that the new req_cfg is smaller than the pending one
and emits "req_cfg regressed from 2 to 1".

Fix this by deriving a stable other-config:datapath-id from
the bridge name in the ADD_BR macro.  This ensures the DPID
never changes when ports are added or removed, preventing
spurious OpenFlow reconnections during tests.

Assisted-by: Claude Opus 4.6, OpenCode
Acked-by: Xavier Simonart <xsimonar@redhat.com>
Signed-off-by: Ales Musil <amusil@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
(cherry picked from commit ef6b575)

Author: almusil

tests/system-common-macros.at

@@ -42,7 +42,10 @@ m4_define([NS_CHECK_EXEC],
 #
 # Expands into the proper ovs-vsctl commands to create a bridge with the
 # appropriate type, and allows additional arguments to be passed.
-m4_define([ADD_BR], [ovs-vsctl _ADD_BR([$1]) -- $2])
+# A stable datapath-id derived from the bridge name is set.
+m4_define([ADD_BR],
+  [dpid=$(printf '%016x' $(echo '$1' | cksum | cut -d ' ' -f1))
+   ovs-vsctl _ADD_BR([$1]) -- set Bridge $1 other-config:datapath-id="$dpid" -- $2])
 
 # ADD_VETH([port], [namespace], [ovs-br], [ip_addr] [mac_addr], [gateway],
 #          [ip_addr_flags] [ip6_addr] [gateway6])