gtest: add scatter to combined calls and use loops (#303)

* gtest: add scatter to combined calls and use loops

* gtest: run validation inside loop

* gtest: revert small element count to 2520

* gtest: fix memory leak in validation

(cherry picked from commit 36935cfbee)

* Fix combined call UT

* Fix memory leak

* Fix alltoallv test


[ROCm/rccl commit: b33a2cac8b]

projects/rccl/test/test_AllReduceAbort.cpp

@@ -114,6 +114,7 @@ namespace CorrectnessTests
             if (idle) pthread_yield();
         }
 
+        free(done);
         HIPCHECK(hipHostFree(fake_head));
         HIPCHECK(hipStreamDestroy(stream));
         dataset.Release();