Fix creation of ffprobe-test.nut on remote targets.

This is not sufficient to run "make fate-ffprobe" on a remote system:
The ffprobe output contains the relative path to the testfile, it is
necessary to run the test from the build directory.
One solution is to use a script like the following as --target-exec:
ssh target "cd /remote/build/directory; $(printf "%q " "$@")"

tests/Makefile

@@ -44,7 +44,7 @@ tests/test_copy.ffmeta: tests/data
 	$(M)cp -f $(SRC_PATH)/tests/test.ffmeta tests/test_copy.ffmeta
 
 tests/data/ffprobe-test.nut: ffmpeg$(EXESUF) tests/test_copy.ffmeta
-	$(M)$(TARGET_EXEC) ./$< \
+	$(M)$(TARGET_EXEC) $(TARGET_PATH)/$< \
         -f lavfi -i "aevalsrc=sin(400*PI*2*t):d=0.125[out0]; testsrc=d=0.125[out1]; testsrc=s=100x100:d=0.125[out2]" \
         -f ffmetadata -i $(TARGET_PATH)/tests/test_copy.ffmeta \
         -flags +bitexact -map 0:0 -map 0:1 -map 0:2 -map_metadata 1 \