determine-test-runners: add a test

I'll add more substantial tests shortly, but let's start with this one.

Library/Homebrew/test/dev-cmd/determine-test-runners_spec.rb

@@ -0,0 +1,8 @@
+# typed: false
+# frozen_string_literal: true
+
+require "cmd/shared_examples/args_parse"
+
+describe "brew determine-test-runners" do
+  it_behaves_like "parseable arguments"
+end