testdata: add test for dropping conf change

This commit adds a test which ensures a config change is not proposed if
the leader has another yet unapplied config change.

Ported from cockroachdb/cockroach#124804

testdata/confchange_drop_if_unapplied.txt

@@ -0,0 +1,59 @@
+# This test verifies that a config change is not proposed if the leader has
+# unapplied config changes. This ensures a safety requirement stated in
+# https://github.com/etcd-io/etcd/issues/7625#issuecomment-489232411
+
+# The check should be performed even if conf change validation is disabled.
+add-nodes 1 voters=(1) index=2 disable-conf-change-validation=true
+----
+INFO 1 switched to configuration voters=(1)
+INFO 1 became follower at term 0
+INFO newRaft 1 [peers: [1], term: 0, commit: 2, applied: 2, lastindex: 2, lastterm: 1]
+
+campaign 1
+----
+INFO 1 is starting a new election at term 0
+INFO 1 became candidate at term 1
+
+stabilize log-level=none
+----
+ok
+
+# Propose one config change. It should be accepted.
+propose-conf-change 1 transition=explicit
+l2 l3
+----
+ok
+
+# The first config change gets appended.
+process-ready 1
+----
+Ready MustSync=true:
+Entries:
+1/4 EntryConfChangeV2 l2 l3
+
+# Propose another config change. It should be rejected, because the first config
+# change hasn't applied on the leader yet.
+propose-conf-change 1
+l4
+----
+INFO 1 ignoring conf change {ConfChangeTransitionAuto [{ConfChangeAddLearnerNode 4}] []} at config voters=(1): possible unapplied conf change at index 4 (applied to 3)
+
+# The new config change is appended to the log as an empty entry.
+stabilize 1
+----
+> 1 handling Ready
+  Ready MustSync=true:
+  HardState Term:1 Vote:1 Commit:4
+  Entries:
+  1/5 EntryNormal ""
+  CommittedEntries:
+  1/4 EntryConfChangeV2 l2 l3
+  INFO 1 switched to configuration voters=(1)&&(1) learners=(2 3)
+> 1 handling Ready
+  Ready MustSync=false:
+  HardState Term:1 Vote:1 Commit:5
+  CommittedEntries:
+  1/5 EntryNormal ""
+  Messages:
+  1->2 MsgApp Term:1 Log:1/4 Commit:4 Entries:[1/5 EntryNormal ""]
+  1->3 MsgApp Term:1 Log:1/4 Commit:4 Entries:[1/5 EntryNormal ""]