The method acp_modules::get_module_infos() needs to use the extension
finder whenever it is looking for a module's info file. While
transitioning to the new extension system, only the initial search for all
module info files was changed to the new system. Due to this it is not
possible to add an extension's acp/mcp/ucp module manually in the ACP.
This patch will always use the extension finder for the acp module's info
files and therefore properly find the needed file. Additionally, the code
has been cleaned up a little bit.

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The tests add 3 different modules. One acp module that should be found
(acp/a_module), one acp module that should not be found (acp/fail_module),
and one mcp module that should work again (mcp/a_module). The modules'
info files had to be included as they were not auto-loaded for some
reason.
There are several test stages. First of, it is tested if the correct mcp
and acp module is returned. Afterwards, the proper loading of specified
modules is tested. One with an existing module and one with a not existing
module. Finally, the test concludes with trying to get the module info of
not existing ucp modules. Other classes like foobar would have also worked
for that check but I decided to use the ucp type of class as that is the
one type missing from the added test modules.

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The possibilities of the first argument have already been covered
previously. The second argument will be covered with an entry that
should exist, an incorrect entry, and the default false entry that
should use the previously set module class. Unfortunately, the third
argument doesn't have an effect in the tests, as the mocked extension
manager will not properly handle enabled/disabled extensions.

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…_infos()

This will now also enable us to test the $use_all_available parameter of
get_module_infos(), which will not only return the module infos for enabled
extensions but also those from disabled extensions.

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…al test

The ACP function test checks the amount of disabled extensions. Due to
the added disabled extension for the tests of the acp_modules method
get_module_infos(), this needed to be increased from 4 to 5.

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