tests: Rename kernel.common.stack_protection_arm_fp_sharing

This commit renames the `kernel.common.stack_protection_arm_fp_sharing`
test to `kernel.common.stack_protection_arm_fpu_sharing`, in order to
align with the recent `CONFIG_FP_SHARING` to `CONFIG_FPU_SHARING`
renaming.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>