feat(tests): Add support for coverage report

This patch enables support for coverage report on the
unit tests by using the gcovr tool.

The coverage report is stored in ${RMM_BUILD_DIR}/$<CONFIG>/coverage

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I5b6a2f3ebf13cbc036c0d64f422ea18f337dd8f5
5 files changed