twister: Update path structure for tests

With the patch a relative path between a test and west's project top
directory is always calculated instead of being relative to zephyr
base. The test names and results structur change by addition of one
level. This way the names/structure conventon for tests from zephyr and
modules is alligned.
For external tests ".." parts are replaced with "external".
This allows to maintain unified folder structure also for results of
external tests.
Without this patch the folder/name allignment is lost for
external and module test becaming depandant on a way how
twister was called.
.

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