Oxid's testing library cannot open file AllTestsUnit.php

for the oxid eshop I'm working on I implemented the Oxid testing library with composer into another directory. The structure is the following:

/var/www/
|
| - html/ <Shop Document Root>
|    |
|    | ...
|    |
|
| - tests/ <Directory where tests reside>
|    |
|    | - vendor/
|    |
|    | ...

From /var/www/ I run tests/vendor/bin/runtests and get the following output:

Cannot open file "AllTestsUnit.php"

I resolved this by adding the absolute path to this file to the appropriate places in runtests but I don't want to use this 'hack' as I don't want to modify the file every time I install the library.

To avoid this I tried putting the path into php.ini's include_path, which by now looks like the following:

include_path=".:/var/www/tests/vendor/oxid-esales/testing-library/"

Did anyone have the same or similar troubles getting the library to work? If not, does anyone else have an idea what I'm doing wrong?

Thanks in advance for any help.