test_formula: add test_facotry_with_fully_qualified_name

Closes Homebrew/homebrew#42550.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/test/test_formula.rb

@@ -228,6 +228,22 @@ class FormulaTests < Homebrew::TestCase
     path.unlink
   end
 
+  def test_factory_with_fully_qualified_name
+    name = "foo-bar"
+    path = HOMEBREW_PREFIX+"Library/Formula/#{name}.rb"
+    path.dirname.mkpath
+    File.open(path, "w") do |f|
+      f << %(
+        class #{Formulary.class_s(name)} < Formula
+          url 'foo-1.0'
+        end
+            )
+    end
+    assert_kind_of Formula, Formulary.factory("homebrew/homebrew/#{name}")
+  ensure
+    path.unlink
+  end
+
   def test_class_specs_are_always_initialized
     f = formula { url "foo-1.0" }