swtbot: Help stabilize SWTChart tests by waiting for number of series

swtbot: Help stabilize SWTChart tests by waiting for number of series

Some tests do not have any waiting for the chart to be populated
before starting to inspect its content. By waiting until the expected
number of series is reached, it is much more likely that the expected
content will be there.

Change-Id: Id0c4f8ec2fae2507ed37c2e3ef9178f789af8b5e
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/75782
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Hudson CI