update test suite for xapian 1.3

[David Bremner]

Without these patches, only two tests fail. Both fixes are fairly
trivial. According to Olly the behviour of xapian 1.3.4 of throwing an
exception rather than optimising away a giant term is a bug, but it is
easy to work around since we aren't really interested in giant terms
per se in the test, just long query strings.

To test this on Debian

apt-get install -t experimental libxapian-dev
XAPIAN_CONFIG=xapian-config-1.3 ./configure
make test

Currently the libxapian-dev packages are not coinstallable (although
the actual libraries are). But downgrading back to stretch version
worked fine for me.