opa test: handle code coverage percentage

Compute the code coverage percentage of each file and the overall one.

Allow the user to specify a coverage threshold when running the `opa test`
command. The program execution will fail when the global file coverage
doesn't respect the required threshold.

`opa test` will exit with error code 2 when the coverage doesn't meet
the specified threshold.

Fixes issue #1029.

Signed-off-by: Flavio Castelli <fcastelli@suse.com>