convolve test: validate 1D filters are 1D

Since the 8-tap lowpass filter is non-interpolating, the results are
different between applying it at whole-pel values and not. This
means that 1D-only versions are requried to be implemented, as
opposed to being an optimization of the 2D case. Calling the 2D
filter instead of the horizontal-only filter is not equivalent
in this case. Update the test to pass invalid filters to the
unused stage of the 1D-only calls, to verify they're unused.

Change-Id: Idc1c490f059adadd4cc80dbe770c1ccefe628b0a