dtc: Fix assorted problems in the testcases for the -a option

We just added the -a option to allow padding of the output dtb's size to
a specified alignment.  Unfortunately the test cases for this had several
bugs:
    * Didn't actually test anything since "alignbase" instead of $alignbase
      was passed to the checker function
    * Introduced an unnecessary run_local_test wrapper
    * Didn't provide very helpful output on failure
    * Only attempted to check one alignment value

This patch fixes up these problems.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>