test: twister: testplan.py use normpath

when load test plan it is possible the plan is built in another os
so the case key would be
'samples/hello_world/samples.baseic.hello_world'
but the testsuite is scaned in current os may in window
and the key is like
'samples\\hello_world\\samples.baseic.hello_world'

so update the uniq path with only backslash in path

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