tests: arch: arm: no_multithreading: Limit platforms allowed

When multithreading is off, kernel source files like sem.c (samphore
implementation) are not present in the build. Some platforms by default
fetch modules or drivers that are using multithreading primitives and
because of that fails to compile when multithreading is off.

Limit the test to only qemu platforms since test is arch specific.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>