Add system-level test of SRAM memory interface

[sgherbst]

Right now we have the tests SRAM and SRAM_MULTI, but they are block-level tests. We need a system-level test to show that we can read out some data from the ADC in the right order. We implemented part of this for ButterPHY in the test LOOPBACK_JTAG, but the test was never passing, so it needs to be updated for DragonPHY and debugged.

[sgherbst]

It looks like this was addressed in cpu_system_tests/loopback_sram, but there is still some leftover code from the old approach. I think we should revisit this to make sure we're checking what we think we're checking.