Test: Merge IntegrationTests into Tests

Consolidate all the tests into a single folders.  This change only moves
the IntegrationTests/Sources and IntegrationTests/Tests to their
respective directory.  In addition, update the IntegratioNTests to call
the respective `executeSwift*()` function and remove its dependency, as
much as possible, on Swift Tools Support Core in favor our the `Basics`
module.

A subsequent change will find better homes for the tests under
Tests/IntegrationTests.

Author: bkhouri