Create Sender and Sender operation test set

[skoppe]

Create a test function that performs common operations against a Sender to detect edge cases. As well as one that calls operations with common Senders.

For instance, in toShared the .connect is called in a nothrow function. .connect isn't required to be nothrow although a lot of them are (or inferred to be). The problem was only found out when we took a SenderObjectBase (which has a virtual connect because it is a class and it isn't nothrow) and ran it through toShared.

Running a SenderObjectBase through a sender operation should be part of the testset.
As well as running toShared on any Sender.