fwk: Add code coverage for framework tests

This patch adds code coverage for framework unit tests. The
output information can be found under `<BUILD_DIR>/framework/test`.
To avoid collisions between framework test code coverage and
module test code coverage, the output directory for module tests
is changed to `<BUILD_DIR>/unit_test` instead of `<BUILD_DIR>`.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I3d2de02c620a2ff4b12c6ff0c3291f9f97bc272f