A subsequent commit will use gegl:path to create images that will be
used as input data for testing. It turns out that, for hitherto
unknown reasons, using multiple threads causes minor disturbances in
gegl:path's output which interferes with the tests. Therefore, it is
useful to override the number of threads used by GEGL for affected
tests where the multi-threading isn't part of the code that is being
tested.

Since gegl_init reads the GEGL_THREADS environment variable, it has to
be called later so that it can override the default GEGL
configuration.

!84