mpih-const-time: fix return value for _gcry_mpih_cmp_ui

* mpi/mpih-const-time.c (_gcry_mpih_cmp_ui): Return '1' when
upper part of MPI is not all zeros.
* tests/mpitests.c (test_invm_cmpui, test_invm): Add tests
for gcry_mpi_invm().

Input such as {U=[0,0,0,1], V=1} should return 1 as U is larger
than V, but function was returning '0'.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>