Create a new fdb-test-utils sub-project for some test utilities

[ScottDugas]

This moves things that were in fdb-extensions and needed for testing out into their own test-specific sub-project. The new project doesn't depend on any of the other sub-projects.

I left MultipleTransactions behind in the testFixtures. It currently depends on FDBError (which is part of fdb-extensions), and currently no other project is depending on it, so I felt that leaving it made sense.

This is in support of: #3574 -- see #3575 (comment)

[ScottDugas]

TeamScale is complaining that we should not use varargs for cartesianProduct.
I think this is a valid usage of varargs, as per the explanation of the issue:

Varargs methods are a convenient way to define methods that require a variable number of arguments, but they should not be overused. They can produce confusing results if used inappropriately.

I think we should tolerate this usage and leave it as-is, but wanted am commenting here as a place for discussion.

I will note that cartesianProduct does have a private overload that takes and returns an incompatible type from the public method.

[ScottDugas]

TeamScale is complaining that we should not use varargs for randomSeeds.
I think this is a valid usage of varargs, as per the explanation of the issue:

Varargs methods are a convenient way to define methods that require a variable number of arguments, but they should not be overused. They can produce confusing results if used inappropriately.

I think we should tolerate this usage and leave it as-is, but wanted am commenting here as a place for discussion.