This is a very rough and ready first pass at comparing gcovr's json
output between two different runs. At the moment it will give you a
file level diff between two runs but hopefully it wont be too hard to
extend to give better insight.

After generating the coverage results you run with something like:

  ./scripts/coverage/compare_gcov_json.py \
    -a ./builds/gcov.config1/coverage.json \
    -b ./builds/gcov.config2/coverage.json

My hope is we can use this to remove some redundancy from testing as
well as evaluate if new tests are actually providing additional
coverage or just burning our precious CI time.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Kautuk Consul <kconsul@linux.vnet.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230403134920.2132362-2-alex.bennee@linaro.org>