Test enhancement about class loading and assertion

[peter279k]

Changed log

• The PluginInstallerTest class namespace should change into Cake\Test\Composer\TestCase\Installer so that it can let composer load testing classes automatically. And the tests/bootstrap.php file can be removed.
• Using the assertSame assertion to make assertion equals checking strict.
• According to the PHPUnit fixtures, it should be the protected function setUp() and protected function tearDown() methods.

[markstory]

Thank you 👍