separate rubocop configuration for tests

As stated in commits e423977, 33e373c, fddebc3:

> Ideally I would've configured the extra `AllowedMethods` to only apply
> to the tests, but I couldn't find a simple way to do that with the
> rubocop configuration. It might be worth extracting a separate rubocop
> configuration for the tests.

Here we do just that.

.rubocop.yml

@@ -72,47 +72,13 @@ Layout/LineLength:
 Gemspec/RequiredRubyVersion:
   Enabled: false
 
-# It can be useful to violate this cop in tests in order to be more explicit
-Lint/UselessTimes:
-  Exclude:
-    - 'test/**/*.rb'
-
-# It can be useful to violate this cop in tests
-Lint/EmptyClass:
-  Exclude:
-    - 'test/**/*.rb'
-
-# It can be useful to violate this cop in tests when testing methods that accept a block
-Lint/EmptyBlock:
-  Exclude:
-    - 'test/**/*.rb'
-
-# There are a mix of styles in the tests and so I don't think it's worth enforcing for now
-Naming/VariableNumber:
-  Exclude:
-    - 'test/**/*.rb'
-
-# These methods from Ruby core are legitimately overridden in the tests
-Style/OptionalBooleanParameter:
-  AllowedMethods:
-    - respond_to?
-    - public_methods
-    - protected_methods
-    - private_methods
-    - public_instance_methods
-    - protected_instance_methods
-    - private_instance_methods
-
 # This cop is useful for required environment variables, but not for optional ones
 Style/FetchEnvVar:
   AllowedVars:
     - MOCHA_RUN_INTEGRATION_TESTS
 
 Naming/FileName:
   ExpectMatchingDefinition: true
-  CheckDefinitionPathHierarchyRoots:
-    - test/unit
-    - test/acceptance
   Exclude:
     - lib/mocha/version.rb
     - lib/mocha/minitest.rb

test/.rubocop.yml

@@ -0,0 +1,33 @@
+inherit_from: ../.rubocop.yml # Inherit from the global configuration
+
+# It can be useful to violate this cop in tests in order to be more explicit
+Lint/UselessTimes:
+  Enabled: false
+
+# It can be useful to violate this cop in tests
+Lint/EmptyClass:
+  Enabled: false
+
+# It can be useful to violate this cop in tests when testing methods that accept a block
+Lint/EmptyBlock:
+  Enabled: false
+
+# There are a mix of styles in the tests and so I don't think it's worth enforcing for now
+Naming/VariableNumber:
+  Enabled: false
+
+# These methods from Ruby core are legitimately overridden in the tests
+Style/OptionalBooleanParameter:
+  AllowedMethods:
+    - respond_to?
+    - public_methods
+    - protected_methods
+    - private_methods
+    - public_instance_methods
+    - protected_instance_methods
+    - private_instance_methods
+
+Naming/FileName:
+  CheckDefinitionPathHierarchyRoots:
+    - test/unit
+    - test/acceptance