Support mailer tests using spec DSL

Improve how mailer tests to resolve mailers from the test name.
Add tests for mailer tests using the minitest spec DSL.

actionmailer/lib/action_mailer/test_case.rb

@@ -49,9 +49,11 @@ def mailer_class
         end
 
         def determine_default_mailer(name)
-          name.sub(/Test$/, '').constantize
-        rescue NameError
-          raise NonInferrableMailerError.new(name)
+          mailer = determine_constant_from_test_name(name) do |constant|
+            Class === constant && constant < ActionMailer::Base
+          end
+          raise NonInferrableMailerError.new(name) if mailer.nil?
+          mailer
         end
       end
 

actionmailer/test/test_test.rb

@@ -26,3 +26,147 @@ def test_set_mailer_class_manual_using_string
     assert_equal TestTestMailer, self.class.mailer_class
   end
 end
+
+describe TestTestMailer do
+  it "gets the mailer from the test name" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe TestTestMailer, :action do
+  it "gets the mailer from the test name" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe TestTestMailer do
+  describe "nested" do
+    it "gets the mailer from the test name" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe TestTestMailer, :action do
+  describe "nested" do
+    it "gets the mailer from the test name" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "TestTestMailer" do
+  it "gets the mailer from the test name" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "TestTestMailerTest" do
+  it "gets the mailer from the test name" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "TestTestMailer" do
+  describe "nested" do
+    it "gets the mailer from the test name" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "TestTestMailerTest" do
+  describe "nested" do
+    it "gets the mailer from the test name" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "AnotherCrazySymbolNameMailerTest" do
+  tests :test_test_mailer
+
+  it "gets the mailer after setting it with a symbol" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "AnotherCrazyStringNameMailerTest" do
+  tests 'test_test_mailer'
+
+  it "gets the mailer after setting it with a string" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "Another Crazy Name Mailer Test" do
+  tests TestTestMailer
+
+  it "gets the mailer after setting it manually" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "Another Crazy Symbol Name Mailer Test" do
+  tests :test_test_mailer
+
+  it "gets the mailer after setting it with a symbol" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "Another Crazy String Name Mailer Test" do
+  tests 'test_test_mailer'
+
+  it "gets the mailer after setting it with a string" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end
+
+describe "AnotherCrazySymbolNameMailerTest" do
+  tests :test_test_mailer
+
+  describe "nested" do
+    it "gets the mailer after setting it with a symbol" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "AnotherCrazyStringNameMailerTest" do
+  tests 'test_test_mailer'
+
+  describe "nested" do
+    it "gets the mailer after setting it with a string" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "Another Crazy Name Mailer Test" do
+  tests TestTestMailer
+
+  describe "nested" do
+    it "gets the mailer after setting it manually" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "Another Crazy Symbol Name Mailer Test" do
+  tests :test_test_mailer
+
+  describe "nested" do
+    it "gets the mailer after setting it with a symbol" do
+      assert_equal TestTestMailer, self.class.mailer_class
+    end
+  end
+end
+
+describe "Another Crazy String Name Mailer Test" do
+  tests 'test_test_mailer'
+
+  it "gets the mailer after setting it with a string" do
+    assert_equal TestTestMailer, self.class.mailer_class
+  end
+end