dur003 tests need available render time of 0.5 to be selective

[nigelmegitt]

The dur003-fail test is now valid. Looking back through the history, it used to have a <p> element not associated with any region, with begin="2.5s". We removed that in 30f48ae and doing so should have made no difference, because that content should have been pruned, resulting in an empty ISD. However, before that element was removed, the sandflow/imscHRM implementation failed validation on that test because it had (wrongly) computed the available render time as 0.5s.

Indeed the render time of the ISD beginning at 3s in dur003-fail should compute to just over 0.5s, and that of dur003-pass to less than 0.5s.

I propose to add back the <p begin="2.5s"... to change the available render time back to 0.5s, in both dur003 tests so that they have the expected behaviours.