SYN-8479: synapse to use xdist for test splitting

[MichaelSquires]

No description provided.

[MichaelSquires]

Note I removed the -s from pytest. Downside: messages to stdout won't be printed in the test output but they're not really useful anyway since they're all intertwined with running workers. Upside: we get percentage completion indicators in the output now, example:

[gw5] [ 15%] PASSED synapse/tests/test_lib_stormlib_env.py::StormLibEnvTest::test_stormlib_env 
synapse/tests/test_lib_stormlib_ethereum.py::EthereumTest::test_storm_ethereum 
[gw3] [ 15%] PASSED synapse/tests/test_lib_modelrev.py::ModelRevTest::test_modelrev_0_2_18 
synapse/tests/test_lib_modelrev.py::ModelRevTest::test_modelrev_0_2_19 
[gw0] [ 15%] PASSED synapse/tests/test_axon.py::AxonTest::test_axon_base 
synapse/tests/test_axon.py::AxonTest::test_axon_blob_v00_v01 
[gw0] [ 16%] PASSED synapse/tests/test_axon.py::AxonTest::test_axon_blob_v00_v01 
synapse/tests/test_axon.py::AxonTest::test_axon_http 
[gw7] [ 16%] PASSED synapse/tests/test_lib_layer.py::LayerTest::test_layer_abrv 
synapse/tests/test_lib_layer.py::LayerTest::test_layer_clone 
[gw5] [ 16%] PASSED synapse/tests/test_lib_stormlib_ethereum.py::EthereumTest::test_storm_ethereum 
synapse/tests/test_lib_stormlib_gen.py::StormLibGenTest::test_stormlib_gen 
[gw2] [ 16%] PASSED synapse/tests/test_lib_coro.py::CoroTest::test_lib_coro_forked 
synapse/tests/test_lib_coro.py::CoroTest::test_lib_coro_parserforked 
[gw2] [ 16%] PASSED synapse/tests/test_lib_coro.py::CoroTest::test_lib_coro_parserforked 

[vEpiphyte]

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