Moves the existing tests into the `ModificationOfParameterWithDefault` subdirectory
which already contained a bunch more tests. In the process, I also removed some
duplicated test cases.
Fixes#1832.
In the taint sink, we add an additional check that the given control-flow node
can indeed point to a value that is mutable. This takes care of the guard on the
type.
If and when we get around to adding configurations for all of the taint
analyses, we may want to implement this as a barrier instead, pruning any steps
that go through a type test where the type is not mutable.
