Add the test case for builtin step() function

Add the test case for builtin step() function

The step function has two kind of prototype:
gentypen step(gentypen edge, gentypen x)
and
gentypen step(float edge, gentypen x)

The first's test name is compiler_step_floatX
The second's test name is compiler_stepf_floatX

Signed-off-by: Junyan He <junyan.he@linux.intel.com>
Reviewed-by: Song, Ruiling <ruiling.song@intel.com>