Disable :main_branch_over_master ff on before suite

There is some quirckiness regarding the disablement of the
:main_branch_over_master ff. At the moment, we're disabling it
on each spec, nevertheless when we have objects wrapped into
`let_it_be` that disablement won't affect it, meaning that
the feature flag will be enabled.

spec/spec_helper.rb

@@ -237,6 +237,7 @@ RSpec.configure do |config|
     # Enable all features by default for testing
     # Reset any changes in after hook.
     stub_all_feature_flags
+    stub_feature_flags(main_branch_over_master: false)
 
     TestEnv.seed_db
   end