Reduction tests

[MrBurmark]

While looking through the reducer code and tests I came across some things potential issues.

• Reducers have an initial value that defaults to T(). Does this make sense for reducers where the identity of the operator does not equal T(). For example doing a min reduce with integers over a set of positive numbers with a default constructed reducer would always result in 0 which is probably not desired and the cause may not be immediately obvious.
• The reducer unit tests don't generate their test files via cmake.
• The reducer unit tests don't test bitwise and/or reducers.
• Some of the reducer unit tests for construction are disabled for gpu policies, don't we want to ensure a consistent interface?
• Some of the reducer unit tests do different things for gpu policies, don't we want to ensure a consistent interface?
• Some of the reducer unit tests use the reductions in forone which is not a supported context, it doesn't go through the same mechanisms that forall does like make_launch_body and it doesn't make a per thread copy of the lambda.

[artv3]

Which reducer unit test are disabled for gpu policies?

[MrBurmark]

https://github.com/LLNL/RAJA/blob/develop/test/unit/reducer/tests/test-reducer-constructors.hpp#L44