Make it easy to skip MRI tests using subprocesses

Author: eregon

test/mri/tests/lib/core_assertions.rb

@@ -31,7 +31,7 @@ def message msg = nil, ending = nil, &default
     end
 
     module CoreAssertions
-      ALLOW_SUBPROCESSES = true # !defined?(::TruffleRuby)
+      ALLOW_SUBPROCESSES = ENV['MRI_TEST_SUBPROCESSES'] != 'false' # !defined?(::TruffleRuby)
 
       require_relative 'envutil'
       require 'pp'

tool/jt.rb

@@ -823,6 +823,7 @@ def help
       jt test mri test/mri/tests/test_find.rb [-- <MRI runner options>]
                                                      run tests in given file, -n option of the runner can be used to further
                                                      limit executed test methods
+        MRI_TEST_SUBPROCESSES=false                  skip MRI tests using subprocesses
       jt retag FILE                                  Remove MRI excludes and re-add as necessary for MRI tests
       ---
       jt test basictest                              run MRI's basictest suite