s2a tests are way too slow

[ejona86]

io.grpc.s2a.handshaker.IntegrationTest is taking 40 seconds * 4 = ~3 minutes and io.grpc.s2a.handshaker.S2AStubTest takes another 20 seconds. I don't know the source of the delay, as it doesn't seem to be S2AStub.

@rmehta19