add a contain operator

[agallou]

This operator is will improve the readability of the tags filter.

Before, we needed to write something like this :

./vendor/bin/atoum -d tests --filter 'not ("needsDatabase" in tags)'

Now, we could write something like this :

./vendor/bin/atoum -d tests --filter 'not (tags contains "needsDatabase")'

[jubianchi]

@agallou could you add a changelog entry please ?

[Hywan]

:+1:

[agallou]

changelog entry added.