When running a test suite, try to change to the directory containing the
executable. This allows running a test suite from any directory, and still
allow it to access its .datax file as well as data files (generally in
tests/data_files) used by individual test cases.
Only implemented on Unix-like systems and on Windows.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>