Improve the library to include classes that could be common to all plugins.

[CrochetFeve0251]

Is your feature request related to a problem? Please describe.
Currently just adding the library is not enough to start testing.

Describe the solution you'd like
A library that includes everything which is needed to start testing and also reduce the need for maintenance on plugins.

For that we will have to pass each traits which are inside wp rocket and not specific to it.

Then each fixture which is from the core should then be moved to the library.

Finally we need to make sure that each library used for test is included within phpunit.
To that we can use phpunit-wp-hooks to facilitate hooks usage.

Finally we need to enhance the read me from the library or a second on how to create a testing env on launchpad documentation.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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