Create sim_if and test submodules

[eine]

Following the same approach as with ui.py, in this PR:

• A vunit.sim_if submodule is created.
• Submodule test is renamed to tests.
• A new submodule names test is created, and all the vunit/test_*.py sources are moved there.
• vunit.ui.vunit is merged into vunit/ui/__init__.py and unused imports are removed.

[kraigher]

I am basically ok with this as it makes the structure better. Regarding test vs tests I think tests should really be outside the vunit folder. Moving it outside also makes it less confusing.

[eine]

Regarding test vs tests I think tests should really be outside the vunit folder. Moving it outside also makes it less confusing.

Good idea! It is done now. BTW, builtins imported simulator_check from vunit.test.common (which is now tests.common). However, I think that vunit should not depend on tests. Hence, I moved three helper functions from tests.common to vunit.sim_if.common.