Downgrade Jest to 24

There's a bug where `toEqual` throws when comparing readonly properties,
which affects our JSX matchers.

jestjs/jest#9575

The bug is fixed in master, but not yet released.

Until then, downgrading to 24.

package.json

@@ -64,8 +64,8 @@
     "google-closure-compiler": "^20200112.0.0",
     "gzip-size": "^5.1.1",
     "jasmine-check": "^1.0.0-rc.0",
-    "jest": "^25.1.0",
-    "jest-diff": "^25.1.0",
+    "jest": "^24.0.0",
+    "jest-diff": "^24.0.0",
     "jest-snapshot-serializer-raw": "^1.1.0",
     "minimatch": "^3.0.4",
     "minimist": "^1.2.0",