Expand tests to check for the actual validatable exception message

This was raising a `FrozenError` on Ruby < 3 where interpolated strings
were considered frozen. This [changed in Ruby 3], since such strings are
dynamic there's no point in freezing them by default.

The test wasn't catching this because `FrozenError` actually inherits
from `RuntimeError`:

>> FrozenError.ancestors
=> [FrozenError, RuntimeError, StandardError, Exception, Object ...]

So the exception check passed. Now we're also checking for the error
message to ensure it raised the exception we really expected there.

Closes #5465

[changed in Ruby 3] https://bugs.ruby-lang.org/issues/17104

test/models/validatable_test.rb

@@ -110,9 +110,12 @@ class ValidatableTest < ActiveSupport::TestCase
   end
 
   test 'should not be included in objects with invalid API' do
-    assert_raise RuntimeError do
+    exception = assert_raise RuntimeError do
       Class.new.send :include, Devise::Models::Validatable
     end
+
+    expected_message = /Could not use :validatable module since .* does not respond to the following methods: validates_presence_of.*/
+    assert_match expected_message, exception.message
   end
 
   test 'required_fields should be an empty array' do