improve rspec config

[senid231]

motivation:

warnings like below are appearing in CI and they are possible false positive tests

WARNING: An expectation of `:now` was set on `nil`. To allow expectations on `nil` and suppress this message, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `true`. To disallow expectations on `nil`, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `false`. Called from spec/support/contexts/stub_calculate_period_current_time.rb:12:in `block (2 levels) in <top (required)>'.