Fix gh_6634 test case

gh_6634_different_log_on_tuple_new_and_free_test.lua verifies that
proper debug message gets into logs for tuple_new() and tuple_delete():
occasionally tuple_delete() printed wrong tuple address. However, still
there are two debug logs: one in tuple_delete() and another one in
memtx_tuple_delete(). So to avoid any possible confusions let's fix
regular expression to find proper log so that now it definitely finds
memtx_tuple_delete().

NO_CHANGELOG=<Test fix>
NO_DOC=<Test fix>

test/box-luatest/gh_6634_different_log_on_tuple_new_and_free_test.lua

@@ -23,7 +23,7 @@ g.test_different_logs_on_new_and_free = function()
         box.cfg{log_level = 5}
     end)
 
-    local str = g.server:grep_log('tuple_new[%w_]*%(%d+%) = 0x%x+$', 1024)
+    local str = g.server:grep_log('memtx_tuple_new[%w_]*%(%d+%) = 0x%x+$', 1024)
     local new_tuple_address = string.match(str, '0x%x+$')
     new_tuple_address = string.sub(new_tuple_address, 3)
 
@@ -34,7 +34,7 @@ g.test_different_logs_on_new_and_free = function()
         box.cfg{log_level = 5}
     end)
 
-    str = g.server:grep_log('tuple_delete%w*%(0x%x+%)', 1024)
+    str = g.server:grep_log('memtx_tuple_delete%w*%(0x%x+%)', 1024)
     local deleted_tuple_address = string.match(str, '0x%x+%)')
     deleted_tuple_address = string.sub(deleted_tuple_address, 3, -2)
     t.assert_equals(new_tuple_address, deleted_tuple_address)