fix(run-multiple): restore plugin loading and per-child reportDir (#5577)

[mirao]

Fixes #5577

Two regressions introduced during the 3.x → 4.x ESM migration caused the testomatio HTML reporter (and any plugin with runInWorker: false) to produce no output when using run-multiple.

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Bug 1 – plugins with runInWorker:false silently skipped in child processes

File: lib/container.js

options.child was used to detect "running inside a worker thread" and skipped plugins whose runInWorker is false (testomatio defaults to false). But run-multiple also sets --child on every forked child process, so those plugins were silently skipped there too — no error, no report.

Fix: replace options.child with !isMainThread (from Node's worker_threads). A run-multiple child is a freshly-forked OS process whose isMainThread is true, so the gate no longer fires. An actual run-workers worker thread has isMainThread === false, so the gate still fires as intended.

Context	options.child	isMainThread	Before (skipped?)	After
run-workers worker thread	truthy (index)	false	skipped ✓	skipped ✓
run-multiple child process	truthy (string)	true	skipped ✗ (bug)	loads ✓
normal run / parent process	falsy	true	loads ✓	loads ✓

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Bug 2 – all children write to the same reportDir, last one wins

File: lib/command/run-multiple.js

3.x run-multiple.js replaced three per-child directory keys before forking each child:

overriddenConfig = replaceValueDeep(overriddenConfig, 'output',    path.join(config.output, outputDir))
overriddenConfig = replaceValueDeep(overriddenConfig, 'reportDir', path.join(config.output, outputDir))  // ← dropped in 4.x
overriddenConfig = replaceValueDeep(overriddenConfig, 'mochaFile', path.join(config.output, outputDir, `${browserName}_report.xml`))

The 4.x port dropped the reportDir line, so every child kept the same shared reportDir value from the config (e.g. output/report) and each child's run overwrote the previous one's HTML file.

Fix: restore the missing replaceValueDeep('reportDir', ...) call so each child receives its own directory — matching 3.x behaviour.

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Result

Before:

output/report/testomatio-report.html   ← only webkit's data (chromium overwritten)

After:

output/dev_chromium_1/testomatio-report.html
output/dev_webkit_2/testomatio-report.html

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Tests

A regression test is added to test/runner/run_multiple_test.js backed by a minimal fixture plugin and config. It verifies both bugs together:

1. A plugin with runInWorker: false is initialised once per child (not zero times).
2. Each child receives a distinct reportDir (not the same shared path).

The test fails against the unfixed code and passes with the fix applied.

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