tests: x86_mmu_api: Fixed testcase crash.

Running this testcase on qemu without userspace enabled it
crashes. The testcase was modifing page table information
of the bss region. This in turn caused some variables to be
inaccessible. Fixed it by moving the page manipulation to a
different location.

GH-5646

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>