[ticket/12170] Assign schema keys in migration helper with data_depth 1

The migration helper currently drops any keys with schema changes that have
the data_depth 1. This change will correctly assign the keys to the steps
array without dropping the keys that might contain important info like the
actual table that should be created.

PHPBB3-12170
[ticket/12170] Test for 12170

PHPBB3-12170
Merge remote-tracking branch 'exreaction/ticket/12170' into ticket/12170
Merge remote-tracking branch 'Marc/ticket/12170' into develop

* Marc/ticket/12170:
  [ticket/12170] Test for 12170
  [ticket/12170] Assign schema keys in migration helper with data_depth 1