wrap "require 'drb/unix'" by bgin-end at test/abstract_unit.rb

Related: #19617, #19187, #19533, #19689, #19675.

'drb/unix' does not exist on mingw.

actionpack/test/abstract_unit.rb

@@ -14,7 +14,11 @@
 end
 
 require 'drb'
-require 'drb/unix'
+begin
+  require 'drb/unix'
+rescue LoadError
+  puts "'drb/unix' is not available"
+end
 require 'tempfile'
 
 PROCESS_COUNT = (ENV['N'] || 4).to_i