fix(junit): fix testsuites time to be sum of all testsuite items

[saitonakamura]

not the total time of execution

Description

This addresses 2 problems, one that is general to junit reporter itself, but more debatable, and the other one is merge-reports specific, but is clearly a bug. Let's start with the first one.

1. I haven't found an official junit.xml spec (and I'm not sure if it exists at all), but this document https://github.com/testmoapp/junitxml?tab=readme-ov-file#complete-junit-xml-example suggests that testsuites.time is an

time Aggregated time of all tests in this file in seconds

which makes sense
But the current implementation uses new Date().getTime() - this._timeStart.getTime(), so like, total execution time, including transform, prepare and so on. Which IMO should be changed to aggregated sum

2. When using --merge-reports --reporter=junit the testsuites.time would be completely wrong (due to it being execution time of reports merging, not the tests times aggregation). This is not a junit-only problem, it also affect text reporters, but I'm not sure what is the correct behaviour for them, so I focused on junit here.

I specifically found it when merging reports, since we calculate mean test time by doing testsuites.time / (testsuites.total - testsuites.skipped), and noticed that mean time suddenly dropped down to nanoseconds (I wish it would, though!)

Xml tests are just best-effort, I'd like to use a proper parser, so let me know if they will suffice or not

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[sheremet-va]

This seems correct to me

[saitonakamura]

Anything else required from my side?