scripts: west_commands: Allow passing source dir when looking for test

Before, only following way of getting test-item build was supported:
`west build -b <BOARD> -T <PATH_TO_SOURCE/TEST_NAME>`
This commit allows to also call it by:
`west build -b <BOARD> <PATH_TO_SOURCE> -T <TEST_NAME>`

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>