Using Regexp.union instead of loop to generate filter regexp

We can't just use filter = Regexp.union(method_names).inspect
because we need to anchor our regexp to avoid this kind of collisions:

  focus
  def test_method; end
  def test_method_with_smthg; end

Without anchors regexp (i.e. /TestClass\#test_method|TestClass\#test_method_with_smthg/)
will match both test methods.

On branch escape_names
Your branch is up-to-date with 'origin/escape_names'.

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   lib/minitest/focus4.rb
	modified:   lib/minitest/focus5.rb