Rename `fs_leak_log` to `fs_leak.log` and move to `tmp`.

Library/Homebrew/dev-cmd/tests.rb

@@ -65,7 +65,7 @@ module Homebrew
 
       Homebrew.failed = !$?.success?
 
-      if (fs_leak_log = HOMEBREW_LIBRARY/"Homebrew/test/fs_leak_log").file?
+      if (fs_leak_log = HOMEBREW_LIBRARY_PATH/"tmp/fs_leak.log").file?
         fs_leak_log_content = fs_leak_log.read
         unless fs_leak_log_content.empty?
           opoo "File leak is detected"

Library/Homebrew/test/testing_env.rb

@@ -43,7 +43,7 @@ module Homebrew
   module FSLeakLogger
     def self.included(klass)
       require "find"
-      @@log = File.open("#{__dir__}/fs_leak_log", "w")
+      @@log = File.open(HOMEBREW_LIBRARY_PATH/"tmp/fs_leak.log", "w")
       klass.make_my_diffs_pretty!
     end