Revert "Disable 2030-long-running-child on gcstress"

This reverts commit 871ce22a54f258e49aaf41c9ca2145afbc7451ea.

Reason for revert: The other CL in the topic fixes the underlying problems.

Change-Id: Ida7e3126bb5879a6515f2c288b982429c759c433
diff --git a/test/knownfailures.json b/test/knownfailures.json
index b56a8dd..f5d51c1 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -1347,11 +1347,5 @@
         "tests": ["2029-contended-monitors"],
         "variant": "interpreter | interp-ac | gcstress | trace",
         "description": ["Slow test. Prone to timeouts."]
-    },
-    {
-        "tests": ["2030-long-running-child"],
-        "variant": "gcstress",
-        "bug": "b/147619421",
-        "description": ["Failure not yet understood."]
     }
 ]