nanokernel: tests: increase stack size from 256 bytes

A recent change modified the fiber stack size of these tests
to 256 bytes, which causes a stack overflow on Nios II. (This
arch has lots of registers)

Increase to 384 bytes, which still compiles well on RAM-constrained
targets like Quark SE SS, Nucleo, etc.

Change-Id: I2152ea9fc1fac693638b8f7a00a6b6628e0c42d3
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>