Change test:unit output format to documentation

This makes rspec print out one line per test, which helps verify
progress is being made when spying on CI jobs. GitHub Actions would not
show dots when you hopped in to view a running job, presumably because
its doing line-based streaming.

tasks/test.rake

@@ -4,6 +4,6 @@ require 'rspec/core/rake_task'
 namespace :test do
   RSpec::Core::RakeTask.new(:unit) do |t|
     t.pattern = "test/unit/**/*_test.rb"
-    t.rspec_opts = "--color"
+    t.rspec_opts = "--color --format documentation"
   end
 end