raise exception on warnings when running tests with inv test (#281)

Previously: When running tests with tox, and if there's a warning
issued, the tests fail. This option is not used in tasks.py (for
"invoke test").

Now: Warnings will fail tests also when running "invoke test" (or:
"inv test").

tasks.py

@@ -86,8 +86,8 @@ def test(c, pdb: bool = False) -> None:
     run = get_run_with_print(c)
     pdb_flag = " --pdb " if pdb else ""
     res = run(
-        f"env -u DBUS_SESSION_BUS_ADDRESS python -m pytest {pdb_flag}--cov-branch "
-        "--cov wakepy --cov-fail-under=100",
+        f"env -u DBUS_SESSION_BUS_ADDRESS python -m pytest -W error {pdb_flag}"
+        "--cov-branch --cov wakepy --cov-fail-under=100",
         ignore_errors=True,
     )
     if res.exited: