Collect test fail reasons in case of nested assertions

Collect test fail reasons in case of nested assertions

RUNNER_ASSERT_MSG macro (and related) throw DPL::Test::TestRunner::TestFailed
exception. If such an exception is thrown during clean-up caused by another
exception and test interruption, termination of tests program happen leaving
no clues about termination cause.

This patch checks if an exception has been thrown earlier and if so,
it doesn't throw next exception but passes exception message to TestRunner.
All such messages passed with addFailReason() method are gathered inside
TestRunner and used during ResultCollection after finishing test and passing
results to collectors.

This patch introduces simple concatrenation method of joining all fail
reasons into single string, that is passed to Collectors.
When collectors API will be ready to receive multiple reasons of single test
failure, another patch can simply pass whole fail results queue.

Change-Id: I1bf582da6526a4187e7c4a53afaeea976cf4bda1