tests: fpu_sharing/generic: workaround crash on qemu_leon3

When running tests/kernel/fpu_sharing/generic on qemu_leon3 with the
new ztest API, a fatal error is raised while test is reported success.
So workaround this by changing the main stack size to 2048.

Signed-off-by: Guo Lixin <lixinx.guo@intel.com>