fix TimeConstraintTest on getPosition()

[theod]

there is a test that doesn't pass :

QVERIFY(constraint->getPosition() == 0.25);

because the position is not updated when setting up the durations (nominal, min and max).
but this is also raising a confusion on what should be the position : does it supposed to be relative to nominal or max duration ?

[jcelerier]

but this is also raising a confusion on what should be the position : does it supposed to be relative to nominal or max duration ?

for me it should be relative to nominal duration : else with infinite max it would not work.

[jcelerier]

let's reopen the issue if there is a problem with the current behaviour at some point