tests: throw in case we encounter unhandled promise rejection

As opposed to just showing a warning that might get eaten in cli when
multple jest child processes work in parallel.
See jestjs/jest#3251
This is a bit unsatifactory as it makes the parent jest process hang in
case there are such occurances.

Bug: T212409

tests/config/setup.ts

@@ -1,3 +1,15 @@
 beforeEach( () => {
 	expect.hasAssertions();
 } );
+
+// Break on unhandled promise rejection (default warning might be overlooked)
+// https://github.com/facebook/jest/issues/3251#issuecomment-299183885
+// However...
+// https://stackoverflow.com/questions/51957531/jest-how-to-throw-an-error-inside-a-unhandledrejection-handler
+if ( typeof process.env.LISTENING_TO_UNHANDLED_REJECTION === 'undefined' ) {
+	process.on( 'unhandledRejection', ( unhandledRejectionWarning ) => {
+		throw unhandledRejectionWarning; // see stack trace for test at fault
+	} );
+	// Avoid memory leak by adding too many listeners
+	process.env.LISTENING_TO_UNHANDLED_REJECTION = 'yes';
+}