fix(tests): Better date definition for flacky test

[julienbourdeau]

Test was failing on main.

Failures:

  1) Subscriptions::FreeTrialBillingService#call with trial ended due to previous subscription with the same external_id sets trial_ended_at
     Failure/Error: expect { service.call }.to change { sub.reload.trial_ended_at }.from(nil).to(sub.trial_end_datetime)
       expected `sub.reload.trial_ended_at` to have changed from nil to 2024-04-16 07:21:40.129012000 +0000, but did not change
     # ./spec/services/subscriptions/free_trial_billing_service_spec.rb:38:in `block (4 levels) in <top (required)>'
     # ./spec/spec_helper.rb:31:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:30:in `block (2 levels) in <top (required)>'

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4546 examples, 1 failure