test: fix flaky test on Faker::IdNumber (#3126)

Author: thdaraujo

test/faker/default/test_faker_id_number.rb

@@ -66,7 +66,7 @@ def test_invalid_south_african_id_number
     sample = @tester.invalid_south_african_id_number
 
     assert_raises Date::Error do
-      Date.parse(south_african_id_number_to_date_of_birth_string(sample))
+      Date.parse(sample[0..5])
     end
   end
 
@@ -276,15 +276,12 @@ def test_danish_id_number_invalid_gender
 
   private
 
-  def south_african_id_number_to_date_of_birth_string(sample)
-    "19#{sample[0..1]}/#{sample[2..3]}/#{sample[4..5]}}"
-  end
-
   def assert_valid_south_african_id_number(sample)
     assert_equal 13, sample.length
     assert_match(/^\d{13}$/, sample)
     assert_include Faker::IdNumber::ZA_CITIZENSHIP_DIGITS, sample[10]
     assert_equal Faker::IdNumber::ZA_RACE_DIGIT, sample[11]
-    assert Date.parse(south_african_id_number_to_date_of_birth_string(sample))
+
+    assert Date.parse(sample[0..5])
   end
 end