Generate unique references in organisation factory

Use Faker's `unique` feature to guarantee unique strings for the
`iati_reference` and `beis_organisation_reference` in the organisation
factory.

We have seen the test suite fail with an intermittent failure, unrelated
to actual code failure, caused by the organisation created failing the
uniqueness check for its `iati_reference`:

```
ActiveRecord::RecordInvalid:
Validation failed: Iati reference Iati reference has already been taken
```

E.g. https://github.com/UKGovernmentBEIS/beis-report-official-development-assistance/actions/runs/6978961311/job/18991313569

We suspect this intermittent failure is caused by Faker's pseudo-random
generator accidentally producing the same string within the same spec.
The IATI reference and BEIS organisation reference are more susceptible
to this, because the randomly generated sequences are only 5 characters
long.

Using Faker's unique feature means it's also a good idea to clear the
record of unique values that have been returned between each spec, to
avoid running out of combinations. (Not statistically likely, but why
take chances?)

spec/factories/organisation.rb

@@ -1,8 +1,8 @@
 FactoryBot.define do
   factory :__organisation do
     sequence(:name) { |n| "#{Faker::Company.name} #{n}" }
-    beis_organisation_reference { Faker::Alphanumeric.alpha(number: 5).upcase! }
-    iati_reference { "GB-GOV-#{Faker::Alphanumeric.alpha(number: 5).upcase!}" }
+    beis_organisation_reference { Faker::Alphanumeric.unique.alpha(number: 5).upcase! }
+    iati_reference { "GB-GOV-#{Faker::Alphanumeric.unique.alpha(number: 5).upcase!}" }
     organisation_type { "10" }
     default_currency { "GBP" }
     language_code { "en" }

spec/support/faker.rb

@@ -0,0 +1,5 @@
+RSpec.configure do |config|
+  config.before(:each) do
+    Faker::Alphanumeric.unique.clear
+  end
+end