tmf: Fix intermittent fail in ProjectModelOutputTest.testListOutputs

This adds a waitUntil/condition that waits until there is the correct
number of children under the analysis.

To reproduce the issue, in TmfAnalysisElement.refreshChildren, add a
Thread.sleep(1000); at the beginning.

Bug: 491823
Change-Id: Iaae81093d90f26403cc58b7286c8691db7db3644
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/83052
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>