try to remove some test skips for MJIT

Eric Wong made some effort to keep compatibility around fd with MJIT.
Also I'm hoping r65279 (and r65280) eliminates major MJIT bugs, so I
want to start solely testing MJIT. Other test skips branched by MJIT
enablement seemed reasonable to me.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65285 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/-ext-/thread_fd_close/test_thread_fd_close.rb

@@ -6,8 +6,6 @@ require 'io/wait'
 class TestThreadFdClose < Test::Unit::TestCase
 
   def test_thread_fd_close
-    skip "MJIT thread is unexpected for this" if RubyVM::MJIT.enabled?
-
     IO.pipe do |r, w|
       th = Thread.new do
         begin

test/ruby/test_autoload.rb

@@ -336,7 +336,6 @@ p Foo::Bar
   end
 
   def test_no_leak
-    skip "This sometimes fails with -DMJIT_FORCE_ENABLE. To be investigated." if RubyVM::MJIT.enabled?
     assert_no_memory_leak([], '', <<~'end;', 'many autoloads', timeout: 30)
       200000.times do |i|
         m = Module.new

test/ruby/test_string.rb

@@ -647,10 +647,6 @@ CODE
   end
 
   def test_crypt
-    if RubyVM::MJIT.enabled?
-      skip "This sometimes fails with -DMJIT_FORCE_ENABLE. This seems important to be fixed..."
-    end
-
     assert_equal(S('aaGUC/JkO9/Sc'), S("mypassword").crypt(S("aa")))
     assert_not_equal(S('aaGUC/JkO9/Sc'), S("mypassword").crypt(S("ab")))
     assert_raise(ArgumentError) {S("mypassword").crypt(S(""))}

test/rubygems/test_gem_util.rb

@@ -5,7 +5,6 @@ require 'rubygems/util'
 class TestGemUtil < Gem::TestCase
 
   def test_class_popen
-    skip "MJIT executes process and it's caught by Process.wait(-1)" if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled?
     assert_equal "0\n", Gem::Util.popen(Gem.ruby, '-e', 'p 0')
 
     assert_raises Errno::ECHILD do