The $(TESTS) variable is defined in file 'tests/list-of-tests.mk',
while the $(XFAIL_TESTS) variable is defined in 'tests/Makefile.am'.
This means that when a new xfailing test is to be added, two files
must be touched; this is suboptimal and slightly confusing.
* tests/Makefile.am (XFAIL_TESTS): Definition moved ...
* tests/list-of-tests.mk: ... here.