Only use representative input arguments in generated tests

[casperboone]

Currently, we generate tests that give all kinds of random input to the 'free variables' of a discovered pattern. However, these free variables can have certain properties in the original project where a pattern is found (e.g. non-null). In the case a variable has a non-null guarantee, but we still give null as input, the test-pattern combination is not representative for real life use.