MIssing file referred to in "Running DevTools Tests" section of TESTING.md

Instructions at https://github.com/flutter/devtools/blob/master/TESTING.md#running-devtools-tests refer to a missing file.

**Executing:**
`./tool/update_flutter_sdk.sh --local`

**Results in the following error:**
`-bash: ./tool/update_flutter_sdk.sh: No such file or directory`

The documentation should be updated to refer to `devtools_tool update-flutter-sdk` which works cross platform.

Open TESTING.md and review the “Running DevTools Tests” section containing ./tool/update_flutter_sdk.sh --local. Replace that reference with devtools_tool update-flutter-sdk, then verify the documented command is the cross-platform one described in the issue.

Written by the indexing model from the issue text.