tests: arch: arm thread swap: fix r0 preservation in the test

We use inline assembly to store the return value of _swap(..)
function directly into r0 (in order to ensure that r4-r11
registers are not touched at this point). But we need to store
the r0 into some global memory, to retain the value until we
check it later in an assert expression, otherwise the
compiler may overwrite r0 in subsequent instructions.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>