Sum JUnit report times across multiple files

What does this MR do and why?

A job that publishes several JUnit XML files reports only the last-parsed file's execution time instead of the sum. This MR fixes the aggregation.

Ci::Build#collect_test_reports! parses every report blob into the same TestSuite, so set_total_time_from_xml assigning that file's time discarded what the previous files contributed. The parser now adds to the running total instead.

Two things to know for review:

• Parallel jobs hit this too, and are the more common trigger. test_suite_name strips the parallel suffix, so rspec 1/5 and rspec 2/5 share one suite and overwrite each other even when each publishes a single file.
• The REST API is affected, not just the UI. The wrong value is persisted to build_report_results.tests_duration, so durations already recorded stay wrong; this fixes new pipelines only.

@s.sebastian.geiger diagnosed this bug and proposed a patch in the issue. This MR builds on that patch. I reworked it against current master and added test coverage, and he is credited as co-author on the commit.

The issue also tracks an unrelated comma-separator parsing bug (time="1,043.988" truncated to 1.0), which is untouched here.

🛠️ with ❤️ at Siemens

References

• Issue: #373063

How to set up and validate locally

1. Seed a job carrying a 2-file JUnit report. The files declare time="10.5" and time="3.25", so the correct total is 13.75.

   Setup script — paste into rails c

   user = User.find_by(username: 'root')
   
   # Isolated demo project, owned by root. Safe to re-run.
   project = Project.find_by_full_path("#{user.namespace.full_path}/junit-multifile-demo") ||
     Projects::CreateService.new(user, name: 'junit-multifile-demo',
                                 namespace_id: user.namespace_id,
                                 visibility_level: Gitlab::VisibilityLevel::PRIVATE).execute
   
   files = ['<testsuite name="a" time="10.5"><testcase name="a1" classname="A" time="4.0"/></testsuite>',
            '<testsuite name="b" time="3.25"><testcase name="b1" classname="B" time="1.25"/></testsuite>']
   
   pipeline = Ci::Pipeline.create!(project: project, ref: 'main', sha: SecureRandom.hex(20),
                                   source: :push, user: user, status: :success)
   stage = Ci::Stage.create!(pipeline: pipeline, project: project, name: 'test', position: 0, status: :success)
   build = Ci::Build.create!(pipeline: pipeline, project: project, ci_stage: stage, ref: pipeline.ref,
                             name: 'junit-demo', stage_idx: 0, scheduling_type: :stage, status: :success,
                             user: user, started_at: 5.minutes.ago, finished_at: 4.minutes.ago)
   
   # Both files go into ONE artifact as concatenated gzip members.
   path = File.join(Dir.mktmpdir, 'junit.xml.gz')
   File.open(path, 'wb') do |out|
     files.each do |xml|
       io = StringIO.new
       gz = Zlib::GzipWriter.new(io)
       gz.write(xml)
       gz.close
       out.write(io.string)
     end
   end
   
   artifact = Ci::JobArtifact.new(job: build, project: project, file_type: :junit, file_format: :gzip)
   File.open(path) { |f| artifact.file = f }
   artifact.save!
   Ci::BuildReportResultService.new.execute(build)
   
   puts "#{Gitlab.config.gitlab.url}/#{project.full_path}/-/pipelines/#{pipeline.id}/test_report"
   puts "#{Gitlab.config.gitlab.url}/#{project.full_path}/-/jobs/#{build.id}/test_report"

2. Open both printed URLs. The duration reads 13.75s on this branch, 3.25s before the fix.

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