[devtools] Avoid db_cache collisions and mis-loading This is done by stopping to share the db_cache between TestDataBaseHandler instances It may seem overkill, but is the only way I could get it to work properly. I think the whole db caching code needs some rework to better work with different db handlers. Closes #4601328

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if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',

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           ask_confirm=False):

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    rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',

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        ask_confirm=False)