TST: Defend against non-trading days as period start in commission test.

A bug in the create_random_simulation_parameters allows the period
start to be a non-trading day.

That bug was causing the commission tests to randomly fail, e.g.
when the period start was on Good Friday, because the commission was
created on hour three of Good Friday, instead of the next Monday.
When it hit that case, the test commission is never processed.

Defend against that bug by using the first open of the simulation
parameters which is more guaranteed to be during market hours,
when creating the test commission.

This is in place of fixing the bug in the random parameters function
or making the parameters non-random, which are other potential fixes.