Run tests from outside the chainer directory

[niboshi]

Rel: #512

If tests are run from within the chainer directory, chainer package installed by pip install ./ won't be imported; instead, source files existing in the directory are imported.

This was a problem in chainer/chainer#7518.

It was tentatively fixed so that chainer is installed in editable mode: #512

We should revert #512 and consider running tests from outside the directory.