Fix test_report_can_be_read to use only successful pipelines

The test test_report_can_be_read was fetching the last pipeline
in the schedule, which corresponds to the currently running test.
Since no test report is generated at that stage, this caused the
nightly build to fail.

This fix ensures that only successful pipelines are considered,
preventing failures due to missing reports.

Signed-off-by: Philip Puk <philip.puk@arm.com>