unit: fix unit test failure on ppc64le (LP: #1271284)

This commit fixes a unit test failure on ppc64le by modifying
the idmap_alloc_next() unit test to allocate a smaller idmap
array ( 24 ), and to exclude the id wrap case.  This is temporary
workaround until the GCC bit-shift operation on ppc64le can be
diagnosed and resolved.

This has been tested on ppc64le and amd64, and should have no effect on production code.