test: set Git name and email.

Extract the code from `bottle.rb` into `utils/git.rb` and ensure it's
run before we run tests.

Should resolve issues like in
https://github.com/Homebrew/homebrew-core/pull/50328

Library/Homebrew/dev-cmd/bottle.rb

@@ -558,15 +558,7 @@ module Homebrew
         end
 
         unless args.no_commit?
-          if ENV["HOMEBREW_GIT_NAME"]
-            ENV["GIT_AUTHOR_NAME"] = ENV["GIT_COMMITTER_NAME"] =
-              ENV["HOMEBREW_GIT_NAME"]
-          end
-
-          if ENV["HOMEBREW_GIT_EMAIL"]
-            ENV["GIT_AUTHOR_EMAIL"] = ENV["GIT_COMMITTER_EMAIL"] =
-              ENV["HOMEBREW_GIT_EMAIL"]
-          end
+          Utils.set_git_name_email!
 
           short_name = formula_name.split("/", -1).last
           pkg_version = bottle_hash["formula"]["pkg_version"]

Library/Homebrew/formula.rb

@@ -1744,6 +1744,7 @@ class Formula
     }
 
     ENV.clear_sensitive_environment!
+    Utils.set_git_name_email!
 
     mktemp("#{name}-test") do |staging|
       staging.retain! if ARGV.keep_tmp?

Library/Homebrew/utils/git.rb

@@ -102,4 +102,12 @@ module Utils
 
     quiet_system "git", "ls-remote", url
   end
+
+  def self.set_git_name_email!
+    return unless ENV["HOMEBREW_GIT_NAME"]
+    return unless ENV["HOMEBREW_GIT_EMAIL"]
+
+    ENV["GIT_AUTHOR_NAME"]  = ENV["GIT_COMMITTER_NAME"]  = ENV["HOMEBREW_GIT_NAME"]
+    ENV["GIT_AUTHOR_EMAIL"] = ENV["GIT_COMMITTER_EMAIL"] = ENV["HOMEBREW_GIT_EMAIL"]
+  end
 end