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

.github/codecov.yml

@@ -4,7 +4,8 @@ codecov:
     after_n_builds: 2		# user and kernel
 
 coverage:
-  precision: 2			# 2 digits of precision
+  precision: 0			# 0 decimals of precision
+  round: nearest        # Round to nearest precision point
   range: "50...90"		# red -> yellow -> green
 
   status:
@@ -20,3 +21,6 @@ comment:
   layout: "reach, diff, flags, footer"
   behavior: once		# update if exists; post new; skip if deleted
   require_changes: yes		# only post when coverage changes
+
+ignore:
+  - "tests/zfs-tests"		# Don't need Tests to cover themselves