formula: move mktemp.

This should be an instance rather than a class method.

Library/Homebrew/formula.rb

@@ -1689,7 +1689,7 @@ class Formula
 
     ENV.clear_sensitive_environment!
 
-    Formula.mktemp("#{name}-test") do |staging|
+    mktemp("#{name}-test") do |staging|
       staging.retain! if ARGV.keep_tmp?
       @testpath = staging.tmpdir
       test_env[:HOME] = @testpath
@@ -1897,6 +1897,16 @@ class Formula
     eligible_for_cleanup
   end
 
+  # Create a temporary directory then yield. When the block returns,
+  # recursively delete the temporary directory. Passing opts[:retain]
+  # or calling `do |staging| ... staging.retain!` in the block will skip
+  # the deletion and retain the temporary directory's contents.
+  def mktemp(prefix = name, opts = {})
+    Mktemp.new(prefix, opts).run do |staging|
+      yield staging
+    end
+  end
+
   private
 
   # Returns the prefix for a given formula version number.
@@ -2465,16 +2475,6 @@ class Formula
       @pour_bottle_check.instance_eval(&block)
     end
 
-    # Create a temporary directory then yield. When the block returns,
-    # recursively delete the temporary directory. Passing opts[:retain]
-    # or calling `do |staging| ... staging.retain!` in the block will skip
-    # the deletion and retain the temporary directory's contents.
-    def mktemp(prefix = name, opts = {})
-      Mktemp.new(prefix, opts).run do |staging|
-        yield staging
-      end
-    end
-
     # @private
     def link_overwrite(*paths)
       paths.flatten!

Library/Homebrew/mktemp.rb

@@ -1,4 +1,4 @@
-# Performs `Formula.mktemp`'s functionality, and tracks the results.
+# Performs `Formula#mktemp`'s functionality, and tracks the results.
 # Each instance is only intended to be used once.
 class Mktemp
   include FileUtils

Library/Homebrew/resource.rb

@@ -82,7 +82,7 @@ class Resource
   # Verifies download and unpacks it
   # The block may call `|resource,staging| staging.retain!` to retain the staging
   # directory. Subclasses that override stage should implement the tmp
-  # dir using FileUtils.mktemp so that works with all subtypes.
+  # dir using Resource#mktemp so that works with all subtypes.
   def stage(target = nil, &block)
     unless target || block
       raise ArgumentError, "target directory or block is required"