swtbot: Make SWTBotTimeGraph/Entry.getEntry() wait for entry

swtbot: Make SWTBotTimeGraph/Entry.getEntry() wait for entry

These methods currently try only once to find the entry but output
"Timed out waiting for..." when it fails.

To match the failure message, and for increased test robustness, make
the methods actually use a wait condition.

Change-Id: Ic1c797b4fa59e44179f0af03fdbb7ae6a27b23bf
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/96240
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI