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_Gather.cpp

@@ -66,6 +66,6 @@ namespace CorrectnessTests
                                 testing::Values(2,3,4,5,6,7,8),
                                 // In-place or not
                                 testing::Values(false),
-                                testing::Values("RCCL_ALLTOALL_KERNEL_DISABLE=0", "RCCL_ALLTOALL_KERNEL_DISABLE=1")),
+                                testing::Values("RCCL_ALLTOALL_KERNEL_DISABLE=1")),
                             CorrectnessTest::PrintToStringParamName());
 } // namespace