Improve Minitest gem template

Default minitest test/ folder structure should be:
- test/test_helper.rb
- test/GEMNAME_test.rb

[fixes rubygems#3512]

lib/bundler/cli/gem.rb

@@ -88,8 +88,8 @@ def run
           )
         when 'minitest'
           templates.merge!(
-            "test/minitest_helper.rb.tt" => "test/minitest_helper.rb",
-            "test/test_newgem.rb.tt" => "test/test_#{namespaced_path}.rb"
+            "test/test_helper.rb.tt" => "test/test_helper.rb",
+            "test/newgem_test.rb.tt" => "test/#{namespaced_path}_test.rb"
           )
         end
       end

lib/bundler/templates/newgem/test/test_newgem.rb.tt → lib/bundler/templates/newgem/test/newgem_test.rb.tt

@@ -1,6 +1,6 @@
-require 'minitest_helper'
+require 'test_helper'
 
-class Test<%= config[:constant_name] %> < Minitest::Test
+class <%= config[:constant_name] %>Test < Minitest::Test
   def test_that_it_has_a_version_number
     refute_nil ::<%= config[:constant_name] %>::VERSION
   end

spec/commands/newgem_spec.rb

@@ -255,8 +255,8 @@ def create_temporary_dir(dir)
       end
 
       it "builds spec skeleton" do
-        expect(bundled_app("test_gem/test/test_test_gem.rb")).to exist
-        expect(bundled_app("test_gem/test/minitest_helper.rb")).to exist
+        expect(bundled_app("test_gem/test/test_gem_test.rb")).to exist
+        expect(bundled_app("test_gem/test/test_helper.rb")).to exist
       end
     end
 
@@ -268,20 +268,20 @@ def create_temporary_dir(dir)
       end
 
       it "builds spec skeleton" do
-        expect(bundled_app("test_gem/test/test_test_gem.rb")).to exist
-        expect(bundled_app("test_gem/test/minitest_helper.rb")).to exist
+        expect(bundled_app("test_gem/test/test_gem_test.rb")).to exist
+        expect(bundled_app("test_gem/test/test_helper.rb")).to exist
       end
 
       it "requires 'test-gem'" do
-        expect(bundled_app("test_gem/test/minitest_helper.rb").read).to include("require 'test_gem'")
+        expect(bundled_app("test_gem/test/test_helper.rb").read).to include("require 'test_gem'")
       end
 
       it "requires 'minitest_helper'" do
-        expect(bundled_app("test_gem/test/test_test_gem.rb").read).to include("require 'minitest_helper'")
+        expect(bundled_app("test_gem/test/test_gem_test.rb").read).to include("require 'test_helper'")
       end
 
       it "creates a default test which fails" do
-        expect(bundled_app("test_gem/test/test_test_gem.rb").read).to include("assert false")
+        expect(bundled_app("test_gem/test/test_gem_test.rb").read).to include("assert false")
       end
     end
 
@@ -499,20 +499,20 @@ def create_temporary_dir(dir)
       end
 
       it "builds spec skeleton" do
-        expect(bundled_app("test-gem/test/test_test/gem.rb")).to exist
-        expect(bundled_app("test-gem/test/minitest_helper.rb")).to exist
+        expect(bundled_app("test-gem/test/test/gem_test.rb")).to exist
+        expect(bundled_app("test-gem/test/test_helper.rb")).to exist
       end
 
       it "requires 'test/gem'" do
-        expect(bundled_app("test-gem/test/minitest_helper.rb").read).to match(/require 'test\/gem'/)
+        expect(bundled_app("test-gem/test/test_helper.rb").read).to match(/require 'test\/gem'/)
       end
 
-      it "requires 'minitest_helper'" do
-        expect(bundled_app("test-gem/test/test_test/gem.rb").read).to match(/require 'minitest_helper'/)
+      it "requires 'test_helper'" do
+        expect(bundled_app("test-gem/test/test/gem_test.rb").read).to match(/require 'test_helper'/)
       end
 
       it "creates a default test which fails" do
-        expect(bundled_app("test-gem/test/test_test/gem.rb").read).to match(/assert false/)
+        expect(bundled_app("test-gem/test/test/gem_test.rb").read).to match(/assert false/)
       end
 
       it "creates a default rake task to run the test suite" do