tests: Add tests for DRAM low power modes

This patch adds two regression tests that execute the script in the
configs dir for triggering low power mode transitions. A separate
test is required for each page policy because for close-adaptive
page policy the DRAM goes into the Precharge Power-down mode while
for open-adaptive page policy it goes into the Activate Power-down
mode.

Change-Id: Iad61af23f132db046f2857cc3ef64b2bf42cf5e4
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5726
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

tests, ruby: Move rubytests from ALPHA (linux) to NULL (none)

This patch avoids compiling ALPHA six times as part of running
'util/regress', and instead relis on NULL with different protocols to
run the rubytest. All we need is the memory system, so there is really
no need to compile the ISA over and over again.

The one downside is the removal of running 'hello' for the variuos
ALPHA and protocol combinations, but if this is a concern we should
rather beef up the synthetic tests for the variuos protocols.