tests: Rename arch.arm.swap.common.fp_sharing

This commit renames the `arch.arm.swap.common.fp_sharing` tests to
`arch.arm.swap.common.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>