[Test] Add a log error formatter

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
[Test] CPUS must be externally defined

The CPUS env variable is used to generate the data extraction scripts and
must match the target system.
Thus, that variable is expected to be defined:
1. at command line
2. or, via the sbox CGroup used to configure the test sandbox

A check is performed at test execution time to verify that this variable
has been properly defined.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
[Test] Generate a local plotting script

The script used to extract and plot all data is now kept on the results
folder for convenience and further checks.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>