kvserver: extend TestStoreRangeMergeRaftSnapshot

This patch extends TestStoreRangeMergeRaftSnapshot to create a
version where the RHS is rebalanced away to different nodes. As
a reminder, the test previously created a scenario where a replica
learned about a bunch of merges and a split via a snapshot, and
validated that it caught up correctly. We now extend the test to
run a variant where the RHS is rebalanced away, validating that the
RHS replica's on-disk state isn't leaked when catching up via the
snapshot.

This test passes as is, and shows that the hazard described in
#73462 is only possible
if there hasn't been a merge. See clearSubsumedReplicaDiskData for the
logic that handles this case.

Epic: none

Release note: None

Author: arulajmani

pkg/kv/kvserver/BUILD.bazel

@@ -576,6 +576,7 @@ go_test(
         "@com_github_stretchr_testify//require",
         "@org_golang_google_grpc//:grpc",
         "@org_golang_google_grpc//metadata",
+        "@org_golang_x_exp//maps",
         "@org_golang_x_sync//errgroup",
         "@org_golang_x_sync//syncmap",
     ],