Move from Scalatest to MUnit

[b-studios]

Moving to munit has a few benefits:

• the library itself is a lot simpler (and might be easier to get into for students)
• the reported diffs for strings are much better (this is especially helpful, since we do a lot of golden-testing)
• it comes with support for ANSI/windows normalized string comparison, which hopefully fixes check files are sensitive to CRLF / LF lineendings #168
• it allows to filter easily based on the file name:
  

There are also a few downsides:

• tests cannot be grouped nicely anymore, like they was with scala tests describe. Instead I am using many different test suites, now.
• do to the missing grouping, test output is not hierarchical anymore and might be harder to read.

For example, here is a run that shows munit:

previously, the tests were grouped by folders.

I am happy to accept these downsides. WDYT?

[jiribenes]

Looks good to me!