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

test/features/step_definitions/setup.rb

@@ -0,0 +1,31 @@
+$index_updated = false # TODO: Hack since Cucumber doesn't have Feature-level hooks
+Given(/^SDKMAN! candidate list is up to date$/) do
+  unless $index_updated
+    run_bash_command('sdk update')
+    $index_updated = true
+  end
+end
+
+Given(/^candidate (\w+) is installed at version (\d+(?:\.\d+)*)$/) do |candidate, version|
+  run_bash_command("sdk install #{candidate} #{version}") unless installed?(candidate, version)
+end
+
+Given(/^candidate (\w+) is installed$/) do |candidate|
+  run_bash_command("sdk install #{candidate}") unless installed?(candidate)
+end
+
+# Uninstall all SDKMAN! candidates
+# TODO: Run after every scenario, this makes tests very slow.
+#       Currently, Cucumber doesn't have Feature-level hooks, so we have to work around:
+#       --> install only if not already installed;
+#           if the test needs a candidate to _not_ be there, make it explicit.
+#       --> clean up after _all_ features at least
+at_exit do
+  Dir["#{ENV['HOME']}/.sdkman/candidates/*/*"].each do |candidate_dir|
+    %r{/([^/]+)/([^/]+)$}.match(candidate_dir) do |match|
+      candidate = match[1]
+      version = match[2]
+      run_bash_command("sdk rm #{candidate} #{version}") unless version == 'current'
+    end
+  end
+end