With subdir-objects, both the top-level makefile, and
libcelt/Makefile, which runs celt-specific unit tests,
were sharing libcelt/.deps for generated dependencies.

Each thought it owned the directory, and so would remove
the files it created in that directory, followed by the
directory itself. The second makefile would then fail to
because the files it created weren't there to remove,
blocking a successful 'make distcheck' with in runs
'make distclean' as part of its verification tests.

We can work around the problem by generating the makefile
for the tests in the actual test directory, so it doesn't
share autotools scratch space with the top level makefile.