Split test and coverage in tasks

Use separate tasks for `test` and `coverage` in invoke's tasks as the former is much
more common and doesn't require coverage and pytest-cov dependencies.

Author: renaud gaudin

tasks.py

@@ -15,6 +15,11 @@ def build_ext(c):
 
 @task
 def test(c):
+    c.run("python -m pytest --color=yes --ff .")
+
+
+@task
+def coverage(c):
     c.run(
         "python -m pytest --color=yes --ff "
         "--cov=libzim --cov-config=.coveragerc "