test: add documentation for common.enoughTestCpu

PR-URL: #31931
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/common/README.md

@@ -68,6 +68,13 @@ sure no unexpected rejections occur, because currently they result in silent
 failures. However, it is useful in some rare cases to disable it, for example if
 the `unhandledRejection` hook is directly used by the test.
 
+### `enoughTestCpu`
+
+* [<boolean>][]
+
+Indicates if there is more than 1 CPU or that the single CPU has a speed of at
+least 1 GHz.
+
 ### `enoughTestMem`
 
 * [<boolean>][]