In MDL-50891, the is_web_crawler() was refactored into a core_useragent
method and the function itself was deprecated. However, there were no
unit tests kept to check the backwards compatible behaviour. It turned
out that the deprecated function leads to PHP fatal error due to a typo.

This patch fixes the typo and brings back the previous unit tests. To be
able to explicitly check the raised debugging message, the test case
now must be subclass of advanced_testcase.

Additionally fixes missing info about the function being deprecated.