Add tests for npy serialization of cfloat types

float16, bfloat16, fp8_e4m3 and fp8_e5m2 are now covered by the
serialize -> deserialize unit tests.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I0a84490586a7e6e4b8190e76164f9d2ff23fd5d0