[ios] Change the test default engine name to avoid confusion

Previously we use "Google" as the test default search eninge name which
could cause confusion by having two "Google" after turning the
experimental policy off.

Bug: 1126318
Change-Id: I8fbce2aeb6180c0097a7179d3b23ae4be0afa212
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412691Reviewed-by: sebsg <sebsg@chromium.org>
Commit-Queue: sebsg <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807184}

ios/chrome/browser/flags/about_flags.mm

@@ -770,7 +770,7 @@ void AppendSwitchesFromExperimentalSettings(base::CommandLine* command_line) {
       base::SysUTF8ToNSString(policy::key::kDefaultSearchProviderSearchURL) :
           @"http://www.google.com/search?q={searchTerms}",
       base::SysUTF8ToNSString(policy::key::kDefaultSearchProviderName) :
-          @"Google",
+          @"TestEngine",
 
       base::SysUTF8ToNSString(policy::key::kPasswordManagerEnabled) : @NO,