Move Mem integration tests into its own Spec

Currently, `src/test/scala/chiselTests/Mem.scala` is filled with unit tests which generate and check CHIRRTL and integration tests which compile to Verilog through MFC and run Verilator to perform a multi-cycle test. Ideally, the integration/simulation tests should be moved in their own file to better separate the CHIRRTL-only tests from them.

Start by reading src/test/scala/chiselTests/Mem.scala and identify which tests generate and check CHIRRTL versus those that compile to Verilog through MFC and run Verilator. Move the integration and simulation tests into a separate spec while keeping the CHIRRTL-only tests in Mem.scala, then run the relevant Mem test suite to verify both groups still pass.

Written by the indexing model from the issue text.