Performance Tests: --runInBand to avoid performance issues in CI envi…

…ronment

Some projects have seen dramatic performance bottlenecks in CI environments
because of the way `jest` parallelizes its test runs and bloats its memory
usage. In this patch we're running the tests in serial to determine if this
sequential-vs-parallel effect is in play in our GitHub Actions runs.

bin/plugin/commands/performance.js

@@ -192,7 +192,7 @@ function curateResults( testSuite, results ) {
  */
 async function runTestSuite( testSuite, performanceTestDirectory, runKey ) {
 	await runShellScript(
-		`npm run test:performance -- packages/e2e-tests/specs/performance/${ testSuite }.test.js`,
+		`npm run test:performance -- --runInBand packages/e2e-tests/specs/performance/${ testSuite }.test.js`,
 		performanceTestDirectory
 	);
 	const resultsFile = path.join(