Fix broken builds due to test that can't find its data

Fix broken builds due to test that can't find its data

LP 817699 reports a problem that began with commit
e9cd992fa4dbf1013346336193cb7469ecee10a8, reintroducing a problem that
had previously been resolved with commit
f017d1261c369b9b5cc3c9cfc4f50d3cdf12a445. In short, tests can and are
run from different directories, but a new test opened a file without
making any allowance that the test might be run from anything other than
the /test/ directory.

This fix uses the dirname(__FILE__) approach to provide the appropriate
relative directory for the test data, and enables the tests to be
processed without dying.

Signed-off-by: Dan Scott <dscott@laurentian.ca>