Closers combines a set of closers, always calling them in order, and using the last as the result.

NewBufconnClient initialises a new gRPC client for testing, using google.golang.org/grpc/test/bufconn.

NewPipeClient initialises a new gRPC client for testing, using a net.Pipe style implementation.

TestConn runs (an implementation of) nettest.TestConn multiple times, testing different permutations of conn order (swapping the roles of c1 and c2), and if closeConns is true, also the order of close (better at catching close-related deadlocks).

DefaultBufconnSize is an arbitrary default.