Fix bug in bug fix in unit test from alignment change (issue #11)

#11

It seems I changed the start and end times based on using half hour
chunks, which align with the start/end times, but forgot to change the
chunk size itself to half an hour. Fixed.

stream_test.go

@@ -94,7 +94,7 @@ func TestStream_Close(t *testing.T) {
 					Groups:   []string{"baz"},
 					Start:    defaultStart,
 					End:      defaultStart.Add(time.Duration(testCase.chunks) * 30 * time.Minute),
-					Chunk:    time.Hour,
+					Chunk:    30 * time.Minute,
 					Priority: si,
 				})
 				require.NoError(t, err, "failed to start stream %d", si)