$ time go test -failfast -short -run 'ΔBTail/rebuild' >x

before:

	real    1m44,975s
	user    2m18,809s
	sys     0m12,384s

after:

	real    1m15,669s
	user    1m31,340s
	sys     0m10,171s

* master:
  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.