fate: fix partial run when no samples path is specified

This restores the behaviour of running only the builtin
regression tests when no path to external samples is given.
aa3805a4 inadvertently broke this by always setting SAMPLES
to something.  Using := makes the ifdef test work as expected
when the FATE_SAMPLES environment variable is empty.

Signed-off-by: Mans Rullgard <mans@mansr.com>