test/dsp: use absolute import path

Avoids "ImportError: attempted relative import with no known parent package"
when doing a simple "python -m unittest test_XXX.py".

artiq/gateware/test/dsp/test_sawg.py

@@ -4,7 +4,7 @@ from migen import *
 from migen.fhdl.verilog import convert
 
 from artiq.gateware.dsp import sawg
-from .tools import xfer
+from artiq.gateware.test.dsp.tools import xfer
 
 
 def _test_gen_dds(dut, o):