Make tests output more concise and clearer

Also refactor the code that generates this output.

This is only a first step, subsequent patches will further improve
the output.

Here is a sample of the old output:

[cpu 0x0000] NOTICE:  Starting unittest 'Template - Single core test'
[cpu 0x0000] NOTICE:  Unittest 'Template - Single core test' complete. Result: Passed
[cpu 0x0000] NOTICE:  Starting unittest 'Template - Multi core test'
[cpu 0x0000] NOTICE:  Unittest 'Template - Multi core test' complete. Result: Passed
========== TEST REPORT ==========
 # Test suite 'Template':
- Single core test: Passed
- Multi core test: Passed
=================================
Tests Skipped : 0
Tests Passed  : 2
Tests Failed  : 0
Tests Crashed : 0
Total tests   : 2
=================================
[cpu 0x0000] NOTICE:  Exiting tests.

And now the new output:

[cpu 0x0000] --
[cpu 0x0000] Running test suite 'Template'
[cpu 0x0000] Description: Template test code
[cpu 0x0000]
[cpu 0x0000] > Executing 'Single core test'
[cpu 0x0000]   TEST COMPLETE                                                 Passed
[cpu 0x0000]
[cpu 0x0000] > Executing 'Multi core test'
[cpu 0x0000]   TEST COMPLETE                                                 Passed
[cpu 0x0000]
[cpu 0x0000] ******************************* Summary *******************************
[cpu 0x0000] > Test suite 'Template'
[cpu 0x0000]                                                                 Passed
[cpu 0x0000] =================================
[cpu 0x0000] Tests Skipped : 0
[cpu 0x0000] Tests Passed  : 2
[cpu 0x0000] Tests Failed  : 0
[cpu 0x0000] Tests Crashed : 0
[cpu 0x0000] Total tests   : 2
[cpu 0x0000] =================================
[cpu 0x0000] NOTICE:  Exiting tests.

Change-Id: I9d52f2da8905962ab1df73d0691846d88622d3b5
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>