tests: Reset transaction synchronizers before every test run
Else, e.g. after a failing test, that closed its storage and DB, but not all Connections, another test, just by starting new transaction, would invoke synchronization on that unclosed connection, which will try to access closed storage and likely fail. Fixes e.g. https://nexedijs.erp5.net/#/test_result_module/20210401-31B27B3D/5 Crash scenariou is the same as described in 5a5ed2c7 (tests: Force-close ZODB connections in teardown, that testing code forgot to explicitly close). Only now we try to isolate tests from each other not only for different modules, but also for tests inside the same module.
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