Prepare v3.20.0

Show DevTools version in status bar tooltip

Fixes Dart-Code#3162.

Add a placeholder Flutter version for when integration_test is suppor…

…ted via "flutter test"

Prepare v3.19.2

Prepare v3.19.1

Prepare v3.19.0

Hide "toggle skipped test" commands from palette

Update test icons

Fixes Dart-Code#3094.

Prepare v3.18.1

Prepare v3.18.0