deps: Upgrade jest to 26.1.0, the latest.

Also, add @jest/source-map, without which some chain of dependencies
doesn't go the right way, and the "modern" fake-timer implementation
isn't available. (That landed in jestjs/jest#7776 with
jestjs/jest@71631f6bf.)

Thankfully, the libdef process was very smooth: there was one
available from FlowTyped, so just `flow-typed install jest@26`. Add
a line in our Flow config under [libs] as suggested in chat [1].

Run `yarn yarn-deduplicate && yarn` as prompted by
`tools/test deps`.

[1] See
    https://chat.zulip.org/#narrow/stream/48-mobile/topic/flow-typed.20for.20Jest/near/854562.
    The particular scenario (where *both* RN's and FlowTyped's Jest
    libdefs are wrong about something) no longer applies. But we
    still need to tell Flow not to use the libdef from RN.

.flowconfig

@@ -48,6 +48,10 @@ flow-typed/react-native-webview_v7.x.x.js
 flow-typed/react-native-url-polyfill_vx.x.x.js
 flow-typed/expo-screen-orientation_vx.x.x.js
 
+; Explicitly name these so that Flow sees them instead of their
+; counterparts in node_modules/react-native:
+flow-typed/npm/jest_v26.x.x.js
+
 
 [strict]
 ; These apply only to files where `@flow strict` is enabled.