license detection: Add the nunit license

[fviernau]

Scanning the nunit license [2] via [1] results in a couple of generic license findings, with ScanCode version 31.1.0, which match only parts of the license text. Add a separate license identifier for that license along with detection capabilities.

[1] https://github.com/apache/avro/blob/6b55656b25cacf0d88cf44d9d802ce46dfaadc83/LICENSE.txt#L312-L345
[2] https://docs.nunit.org/2.6/license.html

[pombredanne]

Thanks for the report!
The latest develop returns something as:

license_expressions:
    - zlib-acknowledgement
    - zlib-acknowledgement
    - zlib

and we should really get only

license_expressions:
    - zlib-acknowledgement
    - zlib-acknowledgement

or just one of these.
Nunit --if I recall correctly-- has a long history (with ScanCode) because of the injection of copyright in the middle of the license text. I have added a note in #2878 too, and your input would mean a lot there too!