Migrate completion tests to Cucumber.

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

test/features/support/helpers.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+def list_installed_candidates
+  candidates = {}
+
+  Dir["#{ENV['HOME']}/.sdkman/candidates/*/*"].each do |candidate_dir|
+    %r{/([^/]+)/([^/]+)$}.match(candidate_dir) do |match|
+      candidate = match[1]
+      version = match[2]
+      candidates[candidate] = [] unless candidates.key?(candidate)
+      candidates[candidate].push version unless version == 'current'
+    end
+  end
+
+  candidates
+end
+
+def installed?(candidate, version = nil)
+  candidates = list_installed_candidates
+  candidates.key?(candidate) && (version.nil? || candidates[candidate].include?(version))
+end
+
+def run_bash_command(cmd)
+  stdout, stderr, status = Open3.capture3("bash -c 'source \"$HOME/.sdkman/bin/sdkman-init.sh\"; #{cmd}'")
+  unless status.success?
+    warn(stderr)
+    raise "Bash command failed: #{stderr}"
+  end
+
+  stdout
+end
+
+def run_fish_command(cmd)
+  # NB: Fish errors out if we don't set terminal dimensions
+  stdout, stderr, status = Open3.capture3("fish -c 'stty rows 80 columns 80; #{cmd}'")
+  unless status.success?
+    warn(stderr)
+    raise 'Fish command failed'
+  end
+
+  stdout
+end