All test: using testify instead of gobottest

[gen2thomas]

This topic was first mentioned in 2018 together with #558 . Meanwhile testify has a powerful set of features and is more widely known.

Removing gobottest will lead to this positive consequences:

• less code to maintain
• more common test style
• much more features available, which will simplify test assertions and writing useful tests

...has this cons:

• for a certain transition period both variants will exist

I would start by mark the gobottest calls deprecated and transform some exemplary tests to the testify format. After that we can try to make a search and replace for all Assert() and Refute() calls (most likely with some more intelligent Regexp modification). All remaining gobottest calls can be modified afterwards step by step.

@deadprogram what do you think?