twister: unify test scenario path to backslash

Winodws user may use the `\` as path, but in twister we use the common `/`
as path separated, to avoid the mis-use, convert it to `/` in twister
first normpath, and then replace the os.sep

tested by:

For Linux Like:
west twister -p disco_l475_iot1 -s samples/hello_world/...

For Windows:
west twister -p disco_l475_iot1 -s samples\hello_world\...

fixing: #70310

Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>