jest: Update all Jest-related dependencies to their latest.

In particular, this gets us Jest 26.4.1, which contains
jestjs/jest#10414. This should fix an issue [1] we'd otherwise
encounter very soon, where differences in a URL object encountered
with `expect(...).toBe(...)` or `expect(...).toEqual(...)` would
result in a very cryptic error message instead of a straightforward
test failure with the desired explanation.

This upgrade was pretty easy to do! I'm hoping fb23341 has
increased the ease of staying up-to-date with Jest.

Since we're going from a 22.x to 23.x of eslint-plugin-jest, we
check the declared breaking changes at 23.0.0 [2]. The only one that
concerns us is that part of the job of `jest/valid-describe` has
been transferred to `jest/valid-title`, and `jest/valid-title` has
been added to the `jest/recommended` config, which we take. The
validation in question is silly, and unfortunately, we can't
reasonably take all of `jest/valid-title` except that part, so, turn
it all off.

See also some prep work for this upgrade in a recent commit.

[1] jestjs/jest#10408
[2] https://github.com/jest-community/eslint-plugin-jest/releases/tag/v23.0.0

.eslintrc.yaml

@@ -165,9 +165,18 @@ rules:
   # don't have a way to do that, and it doesn't make sense to keep these
   # out of the repo.
   jest/no-disabled-tests: off
-  # Only using string literals for test names isn't obviously helpful;
-  # see jest-community/eslint-plugin-jest#203 (rule might be removed?)
-  jest/valid-describe: off
+
+  # Only using string literals for test names, under the heading
+  # `titleMustBeString`, [1] is silly and should probably not be a
+  # thing [2]. We wish we could get all of this rule except that part.
+  # In fact, it's possible to ignore it *for describe blocks
+  # only* [3], but there are no plans to allow turning it off for
+  # `test` blocks.
+  # 
+  # [1] https://github.com/jest-community/eslint-plugin-jest/blob/master/docs/rules/valid-title.md
+  # [2] https://github.com/jest-community/eslint-plugin-jest/issues/203.
+  # [3] https://github.com/jest-community/eslint-plugin-jest/issues/470
+  jest/valid-title: off
 
   # The docs for this rule [1] say that placing an `expect` call
   # outside a `test` or `it` block (e.g., in a `beforeEach` or

package.json

@@ -102,7 +102,7 @@
     "@babel/core": "^7.11.4",
     "@babel/preset-env": "^7.11.0",
     "@babel/runtime": "^7.6.2",
-    "@jest/source-map": "^26.1.0",
+    "@jest/source-map": "^26.3.0",
     "@rollup/plugin-babel": "^5.2.0",
     "@rollup/plugin-node-resolve": "^9.0.0",
     "babel-eslint": "^10.0.3",
@@ -114,17 +114,17 @@
     "eslint-config-prettier": "^6.11.0",
     "eslint-plugin-flowtype": "^4.7.0",
     "eslint-plugin-import": "^2.22.0",
-    "eslint-plugin-jest": "^22.21.0",
+    "eslint-plugin-jest": "^23.20.0",
     "eslint-plugin-jsx-a11y": "^6.3.1",
     "eslint-plugin-prettier": "^3.1.4",
     "eslint-plugin-react": "^7.20.5",
     "eslint-plugin-react-hooks": "^4.0.8",
     "flow-bin": "^0.105.0",
     "flow-coverage-report": "^0.6.0",
     "flow-typed": "^2.4.0",
-    "jest": "^26.1.0",
-    "jest-cli": "^26.1.0",
-    "jest-environment-jsdom": "^26.1.0",
+    "jest": "^26.4.1",
+    "jest-cli": "^26.4.1",
+    "jest-environment-jsdom": "^26.3.0",
     "jest-environment-jsdom-global": "^2.0.4",
     "jest-extended": "^0.11.5",
     "jetifier": "^1.6.5",