Fix test "test get streams" missing initialization - #3376

The test "test get streams" assumes get_streams() would return at least the default CPU stream without any initialisation. This seems to be true before df7f7db, which removed initialization from Scheduler's constructor.

The failure can be reproduced by running the test in isolation:
./build/tests/tests --test-case="test get streams"

Fix by calling default_stream(Device::cpu) before get_streams() to match the test expectation.

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