Makefile: fix bug when running make install under tests/* directory

Makefile: fix bug when running make install under tests/* directory

The TESTS_DIR value is defined in TOPDIR makefile, it is empty here.
So running make install on ext4 directory will get the following info:

../../install-sh -o root -g root -m 755 -d /var/lib/xfstests//ext4

We can see they aren't installed under /var/lib/xfstests/tests/ext4 directory.
Fix this by moving TESTS_DIR definition to include/builddefs.in file.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>