tests: cmsis_dsp: transform: Disable FP16 tests for POSIX arch

This commit disables the half-precision floating-point (FP16) tests for
the POSIX arch, because the `__fp16` type is not supported on the x86
host, which is the most common host type for the POSIX arch.

Note that the FP16 tests can technically run on the POSIX arch if the
host type is AArch64; but, since we do not have a proper configuration
scheme to resolve the host type for the POSIX arch at this time, we
simply disable these tests completely for the POSIX arch.

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