Adapt tests to a change in TFile "file not found" error message

A recent PR in ROOT master changed the way TFile file not found errors are reported.

This MR modifies the affected tests to absorb the difference.

/cc @mato @sailer @ganis

changed milestone to %v35r1

added lhcb-gaudi-head lhcb-lcg-dev3 lhcb-lcg-dev4 lhcb-run2-patches-dev4 labels

• [2021-01-28 00:03] Validation started with lhcb-gaudi-head#2838
• [2021-01-28 00:57] Validation started with lhcb-lcg-dev3#1524
• [2021-01-28 01:06] Validation started with lhcb-lcg-dev4#1538
• [2021-01-28 01:27] Validation started with lhcb-run2-gaudi-head#55
• [2021-01-28 01:57] Validation started with lhcb-run2-patches-dev4#585
• [2021-01-28 02:41] Validation started with lhcb-run2-gaudi-head#55

approved this merge request

merged

mentioned in commit d3917cac