Refs #9630. Added system test for PoldiTruncateData

The test goes through three possible use cases: Too many time bins (the usual raw POLDI data), exactly correct number of bins and too few time bins.
Merge branch 'master' of https://github.com/mantidproject/systemtests into feature/9630_poldi_truncate_data_test
Merge branch 'feature/9630_poldi_truncate_data_test' into develop