run_coverage: prefer coverage3 to python3-coverage

Some of the systems I ran on had a 'python3-coverage' and some had a
'coverage3' program.  More were of the latter name, but more
importantly, the upstream tarball only creates the latter name;
apparently the former was just added by some distros.  So, switch to the
more official name of the program.

Signed-off-by: Elijah Newren <newren@gmail.com>

t/run_coverage

@@ -24,8 +24,8 @@ export PRETEND_UNICODE_ARGS=1
 ls t939*.sh | xargs -n 1 bash
 
 cd $tmpdir
-python3-coverage combine
-python3-coverage html -d $orig_dir/report
-python3-coverage report -m
+coverage3 combine
+coverage3 html -d $orig_dir/report
+coverage3 report -m
 cd $orig_dir
 rm -rf $tmpdir