UtilityMethodTestCase::getTargetToken(): throw a custom exception when target token not found

[jrfnl]

PR #248 introduced the option to throw an exception when the target token couldn't be found (instead of failing the test). In that initial implementation, the exception thrown would be the PHP native RuntimeException.

This commit changes the principle:

• The new parameter $failTest has been removed.
• A new, PHPCSUtils native PHPCSUtils\Exceptions\TestTargetNotFound exception is introduced, which extends the PHP native OutOfBoundsException.
• If the target token is not found, the test will now always throw a TestTargetNotFound exception.

As the change from PR #248 has not been in a release yet, the removal of the parameter is non-breaking.

All the same, this is a functional change as an uncaught exception will result in a test being marked as "errored", while previously, the test would be marked as "failed" when the target token could not be found.

The reason for making this change anyway are as follows:

1. The test run will still be marked as unsuccessful, so in that sense, there is no change.
2. Marking the test as "errored" is semantically more correct as a basic condition for the test to be able to run (having a token to examine) has not been fulfilled.
3. If a test would need to look for different token (contents) depending on PHP/PHPCS versions, they can choose to catch the exception and do a second token search. This means that the benefits of the change from UtilityMethodTestCase::getTargetToken(): add option to throw an exception... #248 are still in place.

Includes perfunctory tests for the new TestTargetNotFound exception. Includes updating the related test in the GetTargetTokenTest class to expect the new exception.