Aggressively reload A/B tests during spec runs

[matthinz]

🛠 Summary of changes

TLDR: This PR updates our RSpec config to reload A/B tests after each spec file in a test run. This is because by default A/B tests are initialized once and only once, and this makes it possible for them to "hold on" to mocked configuration data, potentially leading to flakiness.

Previously in #11583 I added a cleanup hook to the ab_tests_spec.rb file, but there are two problems with this:

1. Theoretically, A/B tests could get "stuck" by any spec, so it needs to be applied more broadly
2. The way I added it didn't actually work. RSpec complained:

WARNING: `after(:suite)` hooks are only supported on the RSpec configuration object. This `after(:suite)` hook, registered on an example group, will be ignored. Called from /builds/lg/identity-idp/spec/config/initializers/ab_tests_spec.rb:6:in `block in <top (required)>'.

Thus, this PR adds the cleanup hook to the main RSpec config. I'm adding it as an after :context block, which here means "after each test file". We could theoretically run the same thing after each example, but that is probably overkill.

📜 Testing Plan

To see an example of this causing a problem on main, run:

bundle exec rspec --seed 4734 --fail-fast \
spec/config/initializers/ab_tests_spec.rb \
spec/features/saml/ial2_sso_spec.rb

ial2_sso_spec.rb will fail.

Switch to this branch, and the same invocation will pass.

[Sgtpluck]

does this add time onto the spec run? (am curious if there's a way to benchmark that.)

[matthinz]

Based on a quick test on my laptop, 1000 A/B test reloads execute in less than 1 second:

it 'tests how fast reloading is' do
    start = Time.zone.now
    1000.times do
      reload_ab_tests
    end
    expect(Time.zone.now - start).to be < 1.second
end

So I don't think this will add any appreciable time to CI runs.