tests/unit_tests: Fix 'make distcheck' with subdir-objects enabled

tests/unit_tests: Fix 'make distcheck' with subdir-objects enabled

Commit 7f72abcf8a56bb35a510a3409e03a4e2aaba50da enabled subdir-objects
when using automake 1.16+.

There is an issue with the handling of .deps directories with this option.
While automake 1.16 fixed subdir-objects to work at all when _SOURCES
contains "unexpanded references" and it did fix subdir-objects to work
with out-of-tree build for "source files specified with an explicit
'$(srcdir)'" those fixes are not transitive. "unexpanded references"
still break out-of-tree builds when enforcing a read-only source dir
like 'make distcheck' does. When using *explicit* references to
srcdir and top_srcdir it works correctly.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20230308150704.128797-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26352.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>