Add custom RuboCop for `assert_not` over `refute`

Since at least cf4afc4 we have preferred `assert_not` methods over
`refute` methods. I have seen plenty of comments in PRs about this,
and we have tried to fix it a few times (5294ad8, e45f176, 8910f12,
41f50be, d4cfd54, 48a183e, and 211adb4), but the `refute` methods
keep sneaking back in.

This custom RuboCop will take care of enforcing this preference, so we
don't have to think about it again. I suspect there are other similar
stylistic preferences that could be solved with some custom RuboCops, so
I will definitely keep my eyes open. `assert_not` over `assert !` might
be a good candidate, for example.

I wasn't totally sure if `ci/custom_cops` was the best place to put
this, but nothing else seemed quite right. At one point I had it set up
as a gem, but I think custom cops like this would have limited value
in another context.

I want to see how code climate handles the new cops before
autocorrecting the existing violations. If things go as expected, I will
push another commit with those corrections.

1 files changed, 35 insertions, 0 deletions

diff --git a/ci/custom_cops/test/support/cop_helper.rb b/ci/custom_cops/test/support/cop_helper.rb
new file mode 100644
index 0000000000..d259154df5
--- /dev/null
+++ b/ci/custom_cops/test/support/cop_helper.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "rubocop"
+
+module CopHelper
+  def inspect_source(cop, source)
+    processed_source = parse_source(source)
+    raise "Error parsing example code" unless processed_source.valid_syntax?
+    investigate(cop, processed_source)
+    processed_source
+  end
+
+  def autocorrect_source(cop, source)
+    cop.instance_variable_get(:@options)[:auto_correct] = true
+    processed_source = inspect_source(cop, source)
+    rewrite(cop, processed_source)
+  end
+
+  private
+    TARGET_RUBY_VERSION = 2.4
+
+    def parse_source(source)
+      RuboCop::ProcessedSource.new(source, TARGET_RUBY_VERSION)
+    end
+
+    def rewrite(cop, processed_source)
+      RuboCop::Cop::Corrector.new(processed_source.buffer, cop.corrections)
+        .rewrite
+    end
+
+    def investigate(cop, processed_source)
+      RuboCop::Cop::Commissioner.new([cop], [], raise_error: true)
+        .investigate(processed_source)
+    end
+end