The reference could be removed because the TEST_DATA_DIR was unused in the
asiodns unit tests. In fact the folder it pointed to did not exist. Removal
of the call to addDataPath allowed for not including and compiling the
dns/tests/unittest_util.h and dns/tests/unittest_util.cc.

The major reason for this change was to remove the dependency on the files
located in the dns/tests because due to the bug in automake they could not
be included in build using an absolute path specified with $(top_srcdir).

Note that when the AUTOMAKE_OPTIONS = subdir-objects is specified, the
object files are created in the subdirectories. When the location of the
source file is specified using a variable like $(top_srcdir) the name of
the variable is treated as a name of the subdirectory, not a path to the
file located out of the tree. Hence, we may need to avoid including files
like this when subdir-objects is in force.