Update validated analyzer warnings in the test runner

[athomas]

This list should be updated:

sdk/pkg/test_runner/lib/src/command_output.dart

Line 439 in 147c919

static const Set<String> _validatedWarnings = {

Context: https://chat.google.com/room/AAAA-IuSyjA/HOThEubstL4/HOThEubstL4?cls=10 (tl;dr: co19 tests had badly formatted language versions, e.g. 3.7.0, and that caused tests to behave incorrectly without failing).

[johnniwinther]

cc @stereotype441