Update QUnit dependency in some packages

This hopefully will end errors like this:
https://github.com/cardstack/boxel/actions/runs/8474124443/job/23219905897#step:9:2947

Author: backspace

packages/realm-server/package.json

@@ -50,7 +50,7 @@
     "prettier": "^2.8.4",
     "prettier-plugin-ember-template-tag": "^1.1.0",
     "qs": "^6.10.5",
-    "qunit": "^2.18.0",
+    "qunit": "^2.20.0",
     "sane": "^5.0.1",
     "start-server-and-test": "^1.14.0",
     "supertest": "^6.2.4",

packages/runtime-common/package.json

@@ -45,7 +45,7 @@
     "marked": "^12.0.1",
     "pluralize": "^8.0.0",
     "qs": "^6.10.5",
-    "qunit": "^2.18.0",
+    "qunit": "^2.20.0",
     "recast": "^0.23.4",
     "safe-stable-stringify": "^2.4.3",
     "super-fast-md5": "^1.0.1",