test: add separate-thread concurrency test #269

Job	Run time
js-test-and-release / check	2m 35s
js-test-and-release / test-node (windows-latest, lts/*)	4m 7s
js-test-and-release / test-chrome-webworker	58s
js-test-and-release / test-firefox	1m 14s
js-test-and-release / test-chrome	1m 1s
js-test-and-release / test-node (ubuntu-latest, lts/*)	53s
js-test-and-release / test-node (macos-latest, lts/*)	4m 53s
js-test-and-release / test-firefox-webworker	1m 20s
js-test-and-release / test-electron-main	59s
js-test-and-release / test-webkit (ubuntu-latest, lts/*)	51s
js-test-and-release / test-webkit (macos-latest, lts/*)	2m 38s
js-test-and-release / test-webkit-webworker (ubuntu-latest, lts/*)	56s
js-test-and-release / test-webkit-webworker (macos-latest, lts/*)	3m 1s
js-test-and-release / test-electron-renderer	25s
js-test-and-release / release-check	0s
js-test-and-release / release	0s
	25m 51s

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