PSMDB-1078: fix KV engine tests for ephemeral storage engines

Update RollingBackToLastStable and CommitBehindStable test cases to handle
ephemeral storage engine behavior where the last stable recovery timestamp
is set immediately when setStableTimestamp is called, unlike non ephemeral
storage engine that require a checkpoint to advance the timestamp.

Author: plebioda

src/mongo/db/storage/kv/kv_engine_test_harness.cpp

@@ -959,7 +959,13 @@ TEST_F(KVEngineTestHarness, RollingBackToLastStable) {
         ASSERT(!engine->getLastStableRecoveryTimestamp());
         ASSERT_EQUALS(engine->getAllDurableTimestamp(), Timestamp(1, 1));
         engine->setStableTimestamp(Timestamp(1, 1), false);
-        ASSERT(!engine->getLastStableRecoveryTimestamp());
+
+        if (engine->isEphemeral()) {
+            // Ephemeral storage engines set the last stable timestamp immediately.
+            ASSERT_EQ(engine->getLastStableRecoveryTimestamp(), Timestamp(1, 1));
+        } else {
+            ASSERT(!engine->getLastStableRecoveryTimestamp());
+        }
 
         // Force a checkpoint to be taken. This should advance the last stable timestamp.
         auto opCtx = _makeOperationContext(engine);
@@ -1026,7 +1032,13 @@ DEATH_TEST_REGEX_F(KVEngineTestHarness, CommitBehindStable, "Fatal assertion.*39
         // Set the stable timestamp to (2, 2).
         ASSERT(!engine->getLastStableRecoveryTimestamp());
         engine->setStableTimestamp(Timestamp(2, 2), false);
-        ASSERT(!engine->getLastStableRecoveryTimestamp());
+
+        if (engine->isEphemeral()) {
+            // Ephemeral storage engines set the last stable timestamp immediately.
+            ASSERT_EQ(engine->getLastStableRecoveryTimestamp(), Timestamp(2, 2));
+        } else {
+            ASSERT(!engine->getLastStableRecoveryTimestamp());
+        }
 
         // Force a checkpoint to be taken. This should advance the last stable timestamp.
         auto opCtx = _makeOperationContext(engine);