tests: use our own generated libtool, not the one in $PATH

tests: use our own generated libtool, not the one in $PATH

libtoolize creates a version of libtool for the right architecture
in $(top_builddir), which is guaranteed to be present, and is
guaranteed to match what we are compiling (even during
cross-compilation).

Packaging systems sometimes separate /usr/bin/libtool, which is
specific to one architecture, from the libtool development files
such as libtoolize and ltmain.sh, which are architecture-independent.
For example, in Debian, libtool_*_all.deb contains the files necessary
to libtoolize a package and is depended on by the dh-autoreconf package,
but libtool-bin_*_amd64.deb (or whatever architecture) contains
/usr/bin/libtool and is not normally necessary to depend on.

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #367
Approved by: cgwalters