(fix): Run any individual test file on it’s own

* I was observing this problem: rspec/rspec-mocks#1079 when running a feature spec in isolation.

```
  1) School viewing public listings when signed in with DfE Sign In A signed in school should see a link back to their own dashboard when viewing public listings
     Failure/Error: validates :minimum_salary, salary: { presence: true, minimum_value: true }

     NoMethodError:

       undefined method `new' for SalaryValidator:Module
```

spec/form_models/job_specification_form_spec.rb

@@ -17,7 +17,7 @@
         end
 
         it 'the minimum salary should be at least equal to the minimum payscale value' do
-          stub_const('SalaryValidator::MIN_SALARY_ALLOWED', '3000')
+          stub_const("#{SalaryValidator}::MIN_SALARY_ALLOWED", '3000')
           expect(job_specification.valid?). to be false
           expect(job_specification.errors.messages[:minimum_salary][0])
             . to eq('must be at least £3000')

spec/models/vacancy_spec.rb

@@ -166,7 +166,7 @@
       end
 
       it 'less than allowed minimum_salary and no maximum_salary' do
-        stub_const('SalaryValidator::MIN_SALARY_ALLOWED', '3000')
+        stub_const("#{SalaryValidator}::MIN_SALARY_ALLOWED", '3000')
         job = build(:vacancy, minimum_salary: '20', maximum_salary: nil)
 
         expect(job.valid?).to be false

spec/rails_helper.rb

@@ -22,7 +22,7 @@
   config.include FactoryBot::Syntax::Methods
 
   config.before do
-    stub_const('SalaryValidator::MIN_SALARY_ALLOWED', '1')
+    stub_const("#{SalaryValidator}::MIN_SALARY_ALLOWED", '1')
   end
 
   config.include ActionView::Helpers::NumberHelper

spec/validators/salary_validator_spec.rb

@@ -62,7 +62,7 @@
 
       context 'if minimum_value: true' do
         it 'validates that the value is not less than the minimum allowed' do
-          stub_const('SalaryValidator::MIN_SALARY_ALLOWED', '400')
+          stub_const("#{SalaryValidator}::MIN_SALARY_ALLOWED", '400')
           model.amount = '12.23'
 
           expect(model).to_not be_valid