Migrate completion tests to Cucumber.

Also retire Fish 2.x + macOS test;
installation of custom brew didn't work out anymore.

test/features/step_definitions/completion.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'open3'
+
+module CompletionHelper
+  def complete(cmd)
+    completions = run_fish_command("complete -C\"sdk #{cmd}\"")
+
+    completions.split("\n") \
+               .map { |line| line.split(/\s+/)[0].strip } \
+               .sort \
+               .uniq \
+               .join(', ')
+    # TODO: Why do we get duplicates in the Docker container?
+    #       --> remove uniq
+  end
+end
+World CompletionHelper
+
+When('the user enters {string} into the prompt') do |cmd|
+  @response = complete(cmd.gsub(/["']/, ''))
+end
+
+Then(/^completion should propose "(.*)"$/) do |completions|
+  expect(@response).to eq(completions)
+end