tests: net: lwm2m: client: Fix stack memory corruptions

LwM2M client context was defined on stack in the test function, however
it could still be in use when the test ended, as the actual LwM2M
teardown took place in a common "after" test function. In result, the
Lwm2M context content could be corrupted.

Additionally, increase the system work queue stack size, as the stack
overflowed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>