RDB AOF preamble: test it in the aofrw unit.

tests/unit/aofrw.tcl

@@ -4,7 +4,9 @@ start_server {tags {"aofrw"}} {
     r config set auto-aof-rewrite-percentage 0 ; # Disable auto-rewrite.
     waitForBgrewriteaof r
 
-    test {AOF rewrite during write load} {
+    foreach rdbpre {yes no} {
+        r config set aof-use-rdb-preamble $rdbpre
+        test "AOF rewrite during write load: RDB preamble=$rdbpre" {
             # Start a write load for 10 seconds
             set master [srv 0 client]
             set master_host [srv 0 host]
@@ -59,6 +61,7 @@ start_server {tags {"aofrw"}} {
             # Make sure they are the same
             assert {$d1 eq $d2}
         }
+    }
 }
 
 start_server {tags {"aofrw"}} {