Require minitest for backward compatibility

Previously ActiveSupport::TestCase was always loaded, so minitest would
be required here:
https://github.com/rails/rails/blob/v7.1.3.2/activesupport/lib/active_support/test_case.rb#L3

Author: eugeneius

lib/rspec/rails/configuration.rb

@@ -1,3 +1,6 @@
+# minitest was previously always loaded, and users may have come to depend on it.
+require "minitest"
+
 # rubocop: disable Metrics/ModuleLength
 module RSpec
   module Rails