Configure Jest to use mutationobserver-shim

Stimulus relies on the MutationObserver browser API to work. Jest's
JSDOM doesn't implement this, so we need to shim it if we want to use
Jest to test our Stimulus controllers.

jest.setup.js

@@ -0,0 +1 @@
+import "mutationobserver-shim";

package.json

@@ -27,12 +27,17 @@
     "jest": "^29.6.1",
     "jest-environment-jsdom": "^29.6.1",
     "jest-fetch-mock": "^3.0.3",
+    "mutationobserver-shim": "^0.3.7",
     "prettier": "^3.0.0"
   },
   "packageManager": "[email protected]",
   "jest": {
     "collectCoverage": true,
+    "setupFilesAfterEnv": [
+      "<rootDir>/jest.setup.js"
+    ],
     "testPathIgnorePatterns": [
+      "app/assets/builds/",
       "node_modules/",
       "tests"
     ],

yarn.lock

@@ -4063,6 +4063,11 @@ [email protected]:
   resolved "https://registry.npmjs.org/ms/-/ms-2.1.2.tgz"
   integrity sha512-sGkPx+VjMtmA6MX27oA4FBFELFCZZ4S4XqeGOXCv68tT+jb3vk/RyaKWP0PTKyWtmLSM0b+adUTEvbs1PEaH2w==
 
+mutationobserver-shim@^0.3.7:
+  version "0.3.7"
+  resolved "https://registry.yarnpkg.com/mutationobserver-shim/-/mutationobserver-shim-0.3.7.tgz#8bf633b0c0b0291a1107255ed32c13088a8c5bf3"
+  integrity sha512-oRIDTyZQU96nAiz2AQyngwx1e89iApl2hN5AOYwyxLUB47UYsU3Wv9lJWqH5y/QdiYkc5HQLi23ZNB3fELdHcQ==
+
 nanomatch@^1.2.9:
   version "1.2.13"
   resolved "https://registry.yarnpkg.com/nanomatch/-/nanomatch-1.2.13.tgz#b87a8aa4fc0de8fe6be88895b38983ff265bd119"