Add multiple negative test generator support

In order to support different generator styles and sets of tests
a new configuration parameter is introduced to define the negative
test generator class. This can be used to define generators that create
only random values (random fuzzy test) or pattern based values.
With this functionality it is also possible to reduce the amount
of negative tests that are automatically produced.

Change-Id: Icfad55d1eea92dc2a42642b37d34c253c26c0846
Partially-implements: bp fuzzy-test
Merge "Add multiple negative test generator support"