tests: use go modules from ./vendor for running tests
tests: use go modules from ./vendor for running tests

Running tests without `-mod=vendor` causes the tests to download the
dependencies if these are not available in the standard go-module
directories (parent directories of the project). All dependencies are
already included in the ./vendor directory, so passing `-mod=vendor`
prevents downloading the dependencies and speeds up testing a lot.

Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi