Improve the assert messages

The assert messages do not make clear which value causes the assert to fail, when more values are used for the test (which means in most of the tests). I tried showing the expected value in the assert message, but when that value is an array or an object, with special characters converted to HTML entities, it is not much easy to read which value caused the failure.

Instead of showing a value as returned by var_export(), it would be much helpful if the key / cache ID shown in the assert message would contain a count number, for example, apc_test_store_and_retrieve_count_01, which would at least make clear the failure is caused by the first tested value.