Fix GroupMessages preprocessor

Fix idempotency of the GroupMessagess preprocessor (introduced in !2260 (merged)). Now it correctly matches lines containing \n (such as after running on its own output).

Also, avoid adding extra new lines characters in grouped messages.

added bug fix testing labels

@mstahl this will fix the failing Moore tests. Locally it works and the only place where we use GroupMessages is in Moore, so we can consider to fast track this in all-slots

assigned to @mstahl

added all-slots label

• [2020-01-14 00:05] Validation started with lhcb-test-throughput2#260
• [2020-01-14 00:06] Validation started with lhcb-lcg-dev3#1145
• [2020-01-14 00:06] Validation started with lhcb-sanitizers#479
• [2020-01-14 00:06] Validation started with lhcb-gaudi-head-py3#277
• [2020-01-14 00:06] Validation started with lhcb-lcg-dev4#1156
• [2020-01-14 00:07] Validation started with lhcb-clang-tidy#47
• [2020-01-14 00:09] Validation started with lhcb-gaudi-head#2502
• [2020-01-14 00:10] Validation started with lhcb-head#2482

resolved all threads

looks good to me, let's merge

removed all-slots label

unassigned @mstahl

merged

mentioned in commit 32f15245

• [2020-01-14 14:36] Validation started with lhcb-test-throughput2#260
• [2020-01-14 16:01] Validation started with lhcb-test-throughput2#260
• [2020-01-14 16:57] Validation started with lhcb-test-throughput2#260
• [2020-01-14 17:55] Validation started with lhcb-test-throughput2#260
• [2020-01-15 11:30] Validation started with lhcb-test-throughput2#260
• [2020-01-15 13:40] Validation started with lhcb-test-throughput2#260
• [2020-01-24 09:44] Validation started with lhcb-test-throughput2#260

mentioned in issue Moore#128 (closed)

added Build label and removed testing label