requirements-dev.txt: use mongomock==2.0.0
requirements-dev.txt: use mongomock==2.0.0

Test regression documents store a list with the sub-group hierarchy.
The mongomock versions before 2.0.0 have a bug in that they fail to
match a document attribute which is a list or a tuple.

Upgrade dev requirements to use 2.0.0 in order to have working unit
tests for regressions.

Signed-off-by: Guillaume Tucker <e7e20b7c30f19d4ca6b81319ef81bf200369d137@collabora.com>

kernelci/kernelci-backend,kernelci/kernelci-backend