Pass stream to device_scalar::value() calls. (#259)

In comms/test.hpp `device_scalar::value` was not being passed an explicit stream, which means that the default stream was being synced. rapidsai/rmm#789 will remove the default from this parameter, and would have therefore broken the RAFT build. So this PR fixes the oversynchronization and ensures RAFT will build after the RMM PR is merged.

Note this PR includes the cmake changes from #258 (just so I could build locally). Once #258 is merged this PR's changes will be simplified.

Authors:
  - Mark Harris (https://github.com/harrism)

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)
  - Chuck Hastings (https://github.com/ChuckHastings)

URL: #259

cpp/include/raft/comms/test.hpp

@@ -378,7 +378,7 @@ bool test_pointToPoint_device_send_or_recv(const handle_t &h, int numTrials) {
 
     communicator.sync_stream(stream);
 
-    if (!sender && received_data.value() != rank - 1) {
+    if (!sender && received_data.value(stream) != rank - 1) {
       ret = false;
     }
 
@@ -424,8 +424,8 @@ bool test_pointToPoint_device_sendrecv(const handle_t &h, int numTrials) {
 
     communicator.sync_stream(stream);
 
-    if (((rank % 2 == 0) && (received_data.value() != rank + 1)) ||
-        ((rank % 2 == 1) && (received_data.value() != rank - 1))) {
+    if (((rank % 2 == 0) && (received_data.value(stream) != rank + 1)) ||
+        ((rank % 2 == 1) && (received_data.value(stream) != rank - 1))) {
       ret = false;
     }