test, postinstall: use WriteMkpathExtension

See https://github.com/Homebrew/brew/pull/20566/files#r2299770459.

Library/Homebrew/postinstall.rb

@@ -12,6 +12,7 @@ require "utils/socket"
 require "cli/parser"
 require "cmd/postinstall"
 require "json/add/exception"
+require "extend/pathname/write_mkpath_extension"
 
 begin
   # Undocumented opt-out for internal use.
@@ -29,6 +30,9 @@ begin
     require "debrew"
     formula.extend(Debrew::Formula)
   end
+
+  Pathname.prepend WriteMkpathExtension
+
   formula.run_post_install
 # Handle all possible exceptions.
 rescue Exception => e # rubocop:disable Lint/RescueException

Library/Homebrew/test.rb

@@ -15,6 +15,7 @@ require "utils/socket"
 require "cli/parser"
 require "dev-cmd/test"
 require "json/add/exception"
+require "extend/pathname/write_mkpath_extension"
 
 DEFAULT_TEST_TIMEOUT_SECONDS = 5 * 60
 
@@ -44,6 +45,8 @@ begin
     formula.extend(Debrew::Formula)
   end
 
+  Pathname.prepend WriteMkpathExtension
+
   ENV.extend(Stdenv)
   ENV.setup_build_environment(formula:, testing_formula: true)