update tests for `dpnp.fft.irfft` and `dpnp.fft.hfft`

[vtavana]

In this PR, tests for dpnp.fft.irfft and dpnp.fft.hfft (which calls back dpnp.fft.irfft) are updated to make sure input array is Hermitian. This means that for 1-D array 0th element of the array has zero imaginary part. In addition, if output array size is N and N is even, the N/2-th element (N/2+1 frequency point) of input (Nyquist mode) also has zero imaginary part.

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