fix border case in test definition

there was the possibility for the random offset added in this test
to add up to a whole frame, which would cause the
re-quantisation to wrap to the next fame (and thus the
CHECK in line 110 to fail.

tests/core/proc/asset/meta/time-grid-basics-test.cpp

@@ -103,8 +103,10 @@ namespace test {
           Time point (myGrid->timeOf (randomFrame));
           CHECK (point == testOrigin + randomFrame * testFps.duration());
           
-          uint fract = 1 + rand() % DIRT_GRAIN;
+          uint fract = 2 + rand() % DIRT_GRAIN;
           FSecs dirt = rational_cast<FSecs> (1 / testFps / fract);
+          ASSERT (Time(dirt) < testFps.duration());
+          ASSERT (0 < dirt);
           
           Time dirty(point + Time(dirt));
           CHECK (point == testOrigin + myGrid->gridAlign(dirty));