Adds verify method in BasePipelineTest.

[nestoracunablanco]

The idea behind is to provide a similar functionality to Mockito.verify: https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#1

When the verification fails, a VerificationException will be thrown, making the explicit assertions in the unit tests less necessary.

Fixes #462

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[nestoracunablanco]

Hi @nre-ableton I see that you are one of the most active developers to this repo. What do you think about this change?
I am just searching for an intuitive way to avoid code duplication by adding a verify method in PipelineTestHelper.
This takes a less ambitious approach than #61 but I think there are some synergies.

[nre-ableton]

The changes look good in general, but I have a few suggestions for improvement.

[nre-ableton]

Thanks!