build-system: remove per-test GCOV reporting

I'm not entirely sure who's using this information and certainly in a
CI environment it just washes over as additional noise. Later patches
will provide new reporting options so a user who wants to analyse
individual tests will be able to use that to get the information.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>