sgi/tests: update access path to sgi_common_util.sh from test scripts

The test scripts under sgi/ directory will be used by other platforms
as well. To allow access to sgi_common_util.sh from test scripts when
tests are invoked from platforms other than sgi/, update the path to
the sgi_common_util.sh script in each of the supported tests.

Change-Id: I3a724aeee77106f8b4692a46df9e1ec882fe9424
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>