[testlib] move repo and related attribute back on Instance instead of Class The repo and cnx was hold by the TestCase class to work as a cache and avoid recreation of the repo from scratch of each test. However since bad26a22fe29 the caching is done by the DatabaseHandler object and it is not necessary have a second layer of cache on the TestCase itself. We move the repo and cnx attribute back on the TestCase instance itself and make several class methods instance methods again. This will helps to change dbapi access in test from dbapi to repoapi. related to #2503918

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