metamorphic: Flush before close if DisableWAL is true

Now that we're using the state of one metamorphic test run
as initial state for future metamorphic test runs with the
crossversion test, we've noticed an incompatibility if the
initial state belonged to a DB with WAL disabled. We'd drop
the mutations since the last flush, as those would not be
written to a WAL or sstable.

Explicitly do a pre-Close Flush() of the db if DisableWAL
is true.

internal/metamorphic/ops.go

@@ -112,6 +112,12 @@ type closeOp struct {
 
 func (o *closeOp) run(t *test, h historyRecorder) {
 	c := t.getCloser(o.objID)
+	if o.objID.tag() == dbTag && t.opts.DisableWAL {
+		// Special case: If WAL is disabled, do a flush right before DB Close. This
+		// allows us to reuse this run's data directory as initial state for
+		// future runs without losing any mutations.
+		_ = t.db.Flush()
+	}
 	t.clearObj(o.objID)
 	err := c.Close()
 	h.Recordf("%s // %v", o, err)