Complete the current test suite

[theofidry]

• Add to the specs the registered classes and functions recorded via the keys 'registered-classes' and 'registered-functions'. This should not change the current behaviour but makes it easier to test this part of the whitelisting feature.
• Add a check to ensure the keys used in 'meta' or a spec definition are all valid
• Add the missing 'registered-functions' where appropriate
• Moved the specs testing from PhpScoperTest to PhpScoperSpecTest to make it more readable
• Do not record the functions return types (which can be identifiers or names as well)