Investigate skipped pp_load_rules unit test

[bjlittle]

As part of a follow-up action of PR #3502, we require to investigate the failing unit test iris.tests.unit.fileformats.pp_load_rules.test__convert_time_coords.TestArrayInputWithLBTIM_0_3_1.test_t1_scalar_t2_list and remove the @unittest.skip decorator.