docbook-xsl autopkg tests fail with libxml/libxslt from unstable:

[...]
  36s autopkgtest [15:41:38]: test test-manpage: [-----------------------
  37s http://www.oasis-open.org/docbook/xml/4.4/dbcentx.mod:324: I/O
error : failed to load
"http://www.oasis-open.org/docbook/xml/4.4/ent/isoamso.ent": loading error
  37s %ISOamso;
  37s          ^
  37s http://www.oasis-open.org/docbook/xml/4.4/dbcentx.mod:324:
warning: failed to load external entity
"http://www.oasis-open.org/docbook/xml/4.4/ent/isoamso.ent"
  37s %ISOamso;
  37s          ^
  37s unable to parse refentry.xml
  37s troff: fatal error: can't open
'/tmp/autopkgtest-lxc.7lad0coa/downtmp/autopkgtest_tmp/refentry.1': No
such file or directory
  37s autopkgtest [15:41:39]: test test-manpage: -----------------------]
  37s autopkgtest [15:41:39]: test test-manpage:  - - - - - - - - - -
results - - - - - - - - - -
  37s test-manpage         FAIL non-zero exit status 1
  37s autopkgtest [15:41:39]: test test-manpage:  - - - - - - - - - -
stderr - - - - - - - - - -
  37s troff: fatal error: can't open
'/tmp/autopkgtest-lxc.7lad0coa/downtmp/autopkgtest_tmp/refentry.1': No
such file or directory
  37s autopkgtest [15:41:39]: test test-book-fop: preparing testbed
  38s Reading package lists...
  38s Building dependency tree...
  38s Reading state information...
  38s Starting pkgProblemResolver with broken count: 0
  38s Starting 2 pkgProblemResolver with broken count: 0
  38s Done
  39s 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
  39s autopkgtest [15:41:41]: test test-book-fop: [-----------------------
  39s http://www.oasis-open.org/docbook/xml/4.5/dbcentx.mod:312: I/O
error : failed to load
"http://www.oasis-open.org/docbook/xml/4.5/ent/isoamsb.ent": loading error
  39s %ISOamsb;
  39s          ^
  39s http://www.oasis-open.org/docbook/xml/4.5/dbcentx.mod:312:
warning: failed to load external entity
"http://www.oasis-open.org/docbook/xml/4.5/ent/isoamsb.ent"
  39s %ISOamsb;
  39s          ^
  39s unable to parse book.xml
  39s I/O warning : failed to load
"/tmp/autopkgtest-lxc.7lad0coa/downtmp/autopkgtest_tmp/book-fop.xml": No
such file or directory
  39s autopkgtest [15:41:41]: test test-book-fop: -----------------------]
  39s autopkgtest [15:41:41]: test test-book-fop:  - - - - - - - - - -
results - - - - - - - - - -
  39s test-book-fop        FAIL non-zero exit status 4
  40s autopkgtest [15:41:42]: test test-book-fop:  - - - - - - - - - -
stderr - - - - - - - - - -
  40s I/O warning : failed to load
"/tmp/autopkgtest-lxc.7lad0coa/downtmp/autopkgtest_tmp/book-fop.xml": No
such file or directory
  40s autopkgtest [15:41:42]: @@@@@@@@@@@@@@@@@@@@ summary
  40s test-manpage         FAIL non-zero exit status 1
  40s test-book-fop        FAIL non-zero exit status 4