test: Ensure tests work if Basher is installed at XDG_DATA_DIR/basher
test: Ensure tests work if Basher is installed at XDG_DATA_DIR/basher

This sets XDG_DATA_HOME in test_Helper.bash to an empty string
so that the tests work if Basher is installed to
XDG_DATA_HOME/basher. Without this change, the tester's current
XDG_DATA_HOME variable value would bleed into the tests, causing many
tests to fail.

basherpm/basher