login/tst-grantpt: Convert to support framework, more error checking

The test now requires working /dev/pts pseudo-terminals.

A new subtest (test_not_ptmx) attempts to call grantpt on a
pseudo-terminal that is not a ptmx device.  POSIX requires an EINVAL
error in this case.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>