unified/test_timer: adapt for unified kernel

Test is poking into microkernel private data structuresa, but the
unified kernel provides an API to retrieve that value.

Abstract the call in the microkernel case to minic the unified kernel
API.

Work by: Dmitriy Korovkin <dmitriy.korovkin@windriver.com>

Change-Id: Ic3195d470fda178164268d9c71c55a2a6daa61a3
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>