tests: cmsis_rtos_v1: remove cast abuses

Casting the tid variable from a void* into an u32_t just to cast it
back to a pointer is pointless. Let's make it a osThreadId variable
upfront and get rid of those casts around it. This also makes it
64-bit compatible.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>