This patch does the following:

1. Make various upgrade related DD tests executable only as big-tests
   when running with valgrind (since they take very long time to complete).

2. Make the call to 'restore_record()' in 'prepare_fields_and_keys()'
   conditional (do it only if TABLE::record[0] != TABLE::s::default_value)
   to avoid the valgrind error. This is necessary since the upgrade from
   5.7 assigns TABLE::record[0] = TABLE::s::default_value).

Change-Id: Ib702439ae8571370fbed6be034b5d0f67b6dd2c5