AbstractFunctionRestrictions sniffs: add tests for namespaced names

[rodrigoprimo]

In preparation for PHPCS 4.0, which changes the tokenization of namespaced names, this PR adds tests with all variants of namespace names (unqualified, partially qualified, fully qualified, and namespace relative) to the sniffs that extend the AbstractFunctionRestrictions class when necessary (some commits already contained part of the required tests).

The only sniff that is missing in this PR is WordPress.Security.EscapeOutput, which will be addressed separately.

Before adding the tests for the WordPress.WP.DeprecatedFunctions sniff, I opted to rename the test case file, as it works differently from the other test case files, and adding the namespaced names to it seemed incorrect.

Note: there will be more sniffs which will need additional tests, so this is only one of the first of a range of PRs adding more tests to help get ready for PHPCS 4.0.

[jrfnl]

Some feedback - not necessarily blocking:

• The "header" comment used Safeguard support for PHP 8.0+ tokenization of namespaced "names". is confusing.
  These tests are not directly related to PHP 8.0, as on PHPCS 3.x, the "old" tokenization is also used for PHP 8.0 - current, while on PHPCS 4.x, the "PHP 8.0 tokenization" is also used for PHP 7.2 - 7.4.
  In my opinion, it would be clearer to call this "Safeguard correct handling of [all types of] namespaced function calls" or "Safeguard against false positives on namespaced function calls".
• All the new tests per sniff use the "same" function name/code. When a sniff is looking for multiple functions, it's never a bad thing to have some variation in the tests.
  Example: DateTime/RestrictedFunctions - the namespaced name tests all use the date_default_timezone_set function name, while the sniff also looks for date().
• For the \target_function() test (FQN call to global function), if the test file in general has enough tests, like the DB/RestrictedFunctions sniff, it would have been perfectly fine to update a few of the pre-existing tests instead of adding a new test for the FQN function call.
• A lot of the test files do not contain any tests with non-lowercase function calls to the target functions. Similar to the previous point - if the test file contains sufficient tests, updating a few of these to be non-lowercase (mixed case, uppercase) would not go amiss.
• A lot of the test files also miss tests against false positives for "same name" method calls/constants/properties etc. While not strictly necessary for PHPCS 4.0, still may not be a bad idea to have these tests.
• The // The sniff should start flagging this once it can resolve relative namespaces. comment seen in quite some of the test files is only partially correct.
  namespace\target_function() will start to be flagged when in the global namespace, but not when within a namespace.
  When namespace and import use tracking is added, we need to make sure both situations are tested (but that's for later, not now).

[jrfnl]

Could these tests have re-used the currently empty block of lines at the top of the file ?

[jrfnl]

These tests need to be expanded a lot more.

This sniff also looks at the parameters for a number of target functions and for those check for certain (global) constants and/or function calls in the parameters, so tests are needed to cover those being fully qualified as well.