sparse-xpow.cc: Improve consistency between sparse and full matrices (bug #63080) sparse-xpow.cc: Return 0x0 matrix for empty inputs of all sizes. Return early for all-zeros matrix with positive power. Add FIXME for all-zeros matrix with negative power. Add BISTs.

private_functions_TEST_FILES = \
  %reldir%/fcn.m \
  %reldir%/private-functions.tst \
  %reldir%/private/ascript.m \
  %reldir%/private/bscript.m \
  %reldir%/private/pfcn1.m \
  %reldir%/private/pfcn2.m

TEST_FILES += $(private_functions_TEST_FILES)