Skip config tests on minimum Rubocop version

While we want to ensure we're always compatible with the minimum Rubocop
version we declare in our gemspec, the config checks are specific to the
Rubocop version listed in Gemfile.lock.

As such, we skip these tests when checking against the minimum Rubocop version.

Rubocop is still invoked against the repository's own code during these runs,
which allows us to catch incompatible config (e.g. cops only available in new
versions, deprecated config keys).

Author: sambostock

.github/workflows/ruby.yml

@@ -28,3 +28,5 @@ jobs:
       run: bundle info rubocop | head -1
     - name: RuboCop and Tests
       run: bundle exec rake
+      env:
+        CHECKING_RUBOCOP_VERSION_COMPATIBILITY: ${{ matrix.gemfile == 'gemfiles/minimum_rubocop.gemfile' }}

test/config_test.rb

@@ -7,6 +7,8 @@
 
 class ConfigTest < Minitest::Test
   def test_config_is_unchanged
+    skip if checking_rubocop_version_compatibility?
+
     Rake.application.load_rakefile
 
     original_config = "test/fixtures/full_config.yml"
@@ -32,6 +34,8 @@ def test_config_is_unchanged
   end
 
   def test_config_has_no_redundant_entries
+    skip if checking_rubocop_version_compatibility?
+
     config = RuboCop::ConfigLoader.load_file("rubocop.yml")
     default_config = RuboCop::ConfigLoader.default_configuration
     redundant_config = Hash.new { |hash, key| hash[key] = {} }
@@ -57,4 +61,10 @@ def test_config_has_no_redundant_entries
 
     assert(redundant_config.empty?, error_message)
   end
+
+  private
+
+  def checking_rubocop_version_compatibility?
+    ENV.fetch("CHECKING_RUBOCOP_VERSION_COMPATIBILITY", "") == "true"
+  end
 end