Split unit and integration tests into separate directories

Unit tests and integration tests are currently located in the same directory. For clarity they should be moved into separated directories

UnitTests
IntegrationTests

Build options should be clarified as well to

ACTS_BUILD_UNIT_TESTS=on
ACTS_BUILD_INTEGRATION_TESTS=on

In preparation for #686.

Edited Jan 21, 2020 by Moritz Kiehn

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