PHP 7.2: test suite errors

simpletest sometimes uses old-style "same name as class" constructors, which PHP 7.2 now warns about. This causes one test failure:

Unexpected PHP Error [Methods with the same name as their class will not be constructors in a future version of PHP; HTMLPurifier_DefinitionTestable has a deprecated constructor] severity [8192] in [simpletest/simpletest/mock_objects.php(1429) : eval()'d code line 1]
in test_get
in HTMLPurifier_DefinitionCache_Decorator_CleanupTest

(Additionally, if running with error_reporting at E_ALL, 4 Deprecated: warnings are printed at test suite startup with the same cause, but those don't cause test failures.)

The underlying issue seems to be in simpletest, so I reported that there as simpletest/simpletest#55

There's also one more "autoload" type problem that appears with E_ALL as well, because tests/common.php has its own separate autoloader. Since it's during test suite startup only, it doesn't cause a failure, though:

Deprecated: __autoload() is deprecated, use spl_autoload_register() instead in HtmlPurifier/tests/common.php on line 10

Start with tests/common.php line 10 and the failing path reported from simpletest/simpletest/mock_objects.php line 1429; review the linked simpletest issue for the old-style constructor behavior. Done means the test suite no longer fails on PHP 7.2 and the E_ALL startup output no longer includes the reported deprecation warnings.