testsuite: ztress: Use XOSHIRO generator when ztress is used

Use XOSHIRO random number generator if target has entropy generator.
Some entropy generators may have limitations (e.g. only thread context)
which would conflict with ztress usage.

Added Kconfig.defconfig for testsuite.

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