Replace hixie@ with @test-exemption-reviewers. (flutter#3325)

More scaleable than listing people's names 1-by-1. Yay!

app_dart/lib/src/service/config.dart

@@ -227,7 +227,7 @@ class Config {
       'request may not have tests. Please make sure to add tests before merging. '
       'If you need '
       '[an exemption](https://github.com/flutter/flutter/wiki/Tree-hygiene#tests) '
-      'to this rule, contact Hixie or stuartmorgan on the #hackers '
+      'to this rule, contact "@text-exemption-reviewers" in the #hackers '
       'channel in [Chat](https://github.com/flutter/flutter/wiki/Chat) '
       '(don\'t just cc them here, they won\'t see it! Use Discord!).'
       '\n\n'

triage_bot/lib/discord.dart

@@ -29,7 +29,7 @@ List<Pattern> get boilerplates => <Pattern>[
   RegExp(r'- \[[ xX]] I listed at least one issue that this PR fixes in the description above\.'),
   RegExp(r'- \[[ xX]] I updated/added relevant documentation \(doc comments with `///`\)\.'),
   RegExp(r'- \[[ xX]] I added new tests to check the change I am making, or this PR is \[test-exempt]\.'),
-  RegExp(r'- \[[ xX]] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt\. See \[testing the engine] for instructions on writing and running engine tests\.'),
+  RegExp(r'- \[[ xX]] I added new tests to check the change I am making or feature I am adding, or @test-exemption-reviewers said the PR is test-exempt\. See \[testing the engine] for instructions on writing and running engine tests\.'),
   RegExp(r'- \[[ xX]] All existing and new tests are passing\.'),
   RegExp(r'- \[[ xX]] The title of the PR starts with the name of the package surrounded by square brackets, e\.g\. `\[shared_preferences]`'),
   RegExp(r'- \[[ xX]] I updated `pubspec.yaml` with an appropriate new version according to the \[pub versioning philosophy], or this PR is \[exempt from version changes]\.'),

triage_bot/test/discord_test.dart

@@ -20,7 +20,7 @@ void main() {
 - [ ] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
 - [ ] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
 - [ ] I listed at least one issue that this PR fixes in the description above.
-- [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
+- [ ] I added new tests to check the change I am making or feature I am adding, or @test-exemption-reviewers said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
 - [ ] I updated/added relevant documentation (doc comments with `///`).
 - [ ] I signed the [CLA].
 - [ ] All existing and new tests are passing.