tox.ini: run linter first

The linter finishes in a few seconds and its errors can be addressed
quickly, whereas the test suite takes a bit longer to run. Let's do
the fast thing first.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

tox.ini

@@ -23,5 +23,5 @@ whitelist_externals =
 setenv =
     TOXTEMPDIR={envtmpdir}
 commands =
-  py.test --cov=west {posargs:tests}
   flake8 --config={toxinidir}/tox.ini {toxinidir}
+  py.test --cov=west {posargs:tests}