test: First basic e2e exchange test:

[dan13bauer]

• Mimicks a source task with a mock partitioned output operator
• Mimicks a sink task with a mock driver that executes the hybrid exchange operator
• The basic test checks whether the correct number of rows arrive at the exchange.

The idea is to add more tests:

• Create multiple source tasks (CudfPartitionedOutputMocks) to test parallelism upstream
• Create multiple exchange tasks to test parallelism at the receiving side
• Increase the number of partitions
• Add remoteSplits while the exchange task is running instead of adding the splits before

[SeanRooooney]

Runs fine, fixed a small error and am extending the types.
Idea to test data consistency by, generating random string AND
then setting the other columns to be derived, i.e. col1=string col2=length of string, col3=hash of string

[SeanRooooney]

Added some attempt to verify the integrity of data, still needs work nut you can run it with:
./_build/release/velox/experimental/cudf-exchange/tests/cudf_exchange_test -v=3 -logtostdout --gtest_filter=CudfExchangeTest.dataTest

[SeanRooooney]

Note one cannot run, multiple tests within the same process as something in ucxx is not released properly, on 2nd test:

terminate called after throwing an instance of 'ucxx::BusyError'
what(): Device is busy