determine-test-runners: fix generic OS test failures

Without this, testing on the generic OS results in errors like

    Error: uninitialized constant Homebrew::MacOS

Library/Homebrew/os.rb

@@ -55,6 +55,7 @@ module OS
   LINUX_PREFERRED_GCC_COMPILER_FORMULA = "gcc@11" # https://packages.ubuntu.com/jammy/gcc
   LINUX_PREFERRED_GCC_RUNTIME_FORMULA = "gcc"
 
+  require "os/generic"
   if OS.mac?
     require "os/mac"
     # Don't tell people to report issues on unsupported configurations.

Library/Homebrew/os/generic.rb

@@ -0,0 +1,8 @@
+# typed: strict
+# frozen_string_literal: true
+
+module OS
+  module Mac
+    ::MacOS = OS::Mac
+  end
+end

Library/Homebrew/os/linux.rb

@@ -54,8 +54,6 @@ module OS
   module Mac
     module_function
 
-    ::MacOS = OS::Mac
-
     raise "Loaded OS::Linux on generic OS!" if ENV["HOMEBREW_TEST_GENERIC_OS"]
 
     def version

Library/Homebrew/os/mac.rb

@@ -13,8 +13,6 @@ module OS
 
     module_function
 
-    ::MacOS = OS::Mac
-
     raise "Loaded OS::Mac on generic OS!" if ENV["HOMEBREW_TEST_GENERIC_OS"]
 
     VERSION = ENV.fetch("HOMEBREW_MACOS_VERSION").chomp.freeze