Do not use email address as Gaia id in Assistant browser tests

Gaia id should never be an email address, and there is a dcheck to guard
against this:
https://source.chromium.org/chromium/chromium/src/+/master:google_apis/gaia/core_account_id.cc;l=34

I have no idea why this was not triggering previously.

Bug: None
Test: browser_tests --gtest_filter="Assistant*"
Change-Id: I47775ec00afd67ea31d92ea09bdf02267ba06672
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548294
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Auto-Submit: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829061}

chrome/browser/ui/ash/assistant/assistant_test_mixin.cc

@@ -39,7 +39,7 @@ namespace assistant {
 namespace {
 
 constexpr const char kTestUser[] = "test_user@gmail.com";
-constexpr const char kTestUserGaiaId[] = "test_user@gaia.id";
+constexpr const char kTestUserGaiaId[] = "test_user_gaia_id";
 
 LoginManagerMixin::TestUserInfo GetTestUserInfo() {
   return LoginManagerMixin::TestUserInfo(