tests: fix spurious failure of cond29.test

tests: fix spurious failure of cond29.test

* tests/cond29.test: Limit the amount of virtual memory available
to the automake process to ~ 150 MB, rather than only ~ 20 MB, to
account for higher (but still acceptable) memory usages on some
systems (in this case, a Fedora 16 distro on ppc64).  To be
sure not to reduce coverage, increase the number of potential
combinations of automake conditionals from 2**22 = 4194304 to
2**24 = 16777216.

We have actually verified that the new version of the test case
catches the Automake 1.7 it is intended to check against, using
the Automake 1.7.9 tarball downloaded from:
  <http://ftp.gnu.org/gnu/automake/automake-1.7.9.tar.gz>

Reported by Jim Meyering in automake bug#10374.