erp5_core_test: Make testWorkflowHistoryList.TestDedup stable.

The precise number of entries in a bucket depend on an estimation of the
size of a pickle. The pickled data contains DateTime objects, making an
equality test brittle:
- DateTime's timezones are stored as strings (ex: 'GMT') whose length
  depend on Zope's timezone, which is variable
- DateTime's time is stored as a floating-point value represented as a
  string whose length depends on the number of units and decimals are
  necessary to represent its value, both being variable (one based on when
  the test was run, the other based on clock precision and exact test
  execution timing)
Instead, restore the originally-considered-acceptable boundary (24) and
verify that the generated value is greater or equal to it.
If 24 is considered too small to be acceptable, then it is a decision
independent from the present change.