Codecov tweaks

Codecov tweaks

Modify the Codecov settings to provide a more realistic and stable
report.  The following change were made:

- Precision has been limited to whole percents only, but will round
  to nearest. This means 0.0-0.49 will round to zero (no change) and
  0.51 will round to 1%.

- Exclude the tests/zfs-tests directory from the report.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>
Closes #9650