test: add separate-thread concurrency test #295

Job	Run time
js-test-and-release / check	2m 13s
js-test-and-release / test-chrome-webworker	1m 0s
js-test-and-release / test-chrome	1m 9s
js-test-and-release / test-node (windows-latest, lts/*)	3m 27s
js-test-and-release / test-node (ubuntu-latest, lts/*)	45s
js-test-and-release / test-firefox	1m 0s
js-test-and-release / test-firefox-webworker	1m 4s
js-test-and-release / test-node (macos-latest, lts/*)	2m 1s
js-test-and-release / test-webkit (ubuntu-latest, lts/*)	1m 3s
js-test-and-release / test-webkit (macos-latest, lts/*)	1m 52s
js-test-and-release / test-webkit-webworker (ubuntu-latest, lts/*)	43s
js-test-and-release / test-electron-main	56s
js-test-and-release / test-webkit-webworker (macos-latest, lts/*)	2m 27s
js-test-and-release / test-electron-renderer	24s
js-test-and-release / release-check	0s
js-test-and-release / release	0s
	20m 4s

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