[test_runner] Fix case handling in update_static_error_tests.

Fixes `update_static_error_tests.dart` to comvert diagnostic codes
from the analyzer to upper case.

This is necessary because the analyzer now reports diagnostic codes
using `lower_snake_case`, but test runner expectations still use
`UPPER_SNAKE_CASE`. Without this change, running
`update_static_error_tests.dart` on a test results in expectations
that will not pass.

Note: I also investigated the possibility of changing the test runner
so that it matches analyzer error codes in a case-insensitive fashion
(as the analyzer itself does when matching `ignore` comments, for
example). But this would have been a lot more effort, because the
logic in the test runner for matching error codes is in several
places, and in many cases shared with logic for matching CFE error
message text (which IMHO should remain case sensitive).

Change-Id: I6a6a696477c1544d02d9832390439c333d768a70
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/478700
Reviewed-by: Kallen Tu <kallentu@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>