Update tests with a (compile time) option SUBSURFACE_TEST_DATA,
pointing to test data base path. It is needed for cross compilation cases.
SUBSURFACE_TEST_DATA is set to SUBSURFACE_SOURCE by default,
or configurable via cmake option -DSUBSURFACE_TEST_DATA="...".
Signed-off-by: Jeremie Guichard <djebrest@gmail.com>