Move an unstable bootstraptest to pending

This has been unstable on AppVeyor mswin since the introduction
3fd83cb6fcc483d2eac0795bc139c521a3a59bd2.
https://ci.appveyor.com/project/ruby/ruby/builds/27103307/job/j7xwjmsos2k22cck

Let's have it in pending.rb to be fixed.

bootstraptest/pending.rb

@@ -37,3 +37,8 @@ assert_normal_exit %q{
   r.instance_eval { initialize r, r }
   r.inspect
 }
+
+# This randomly fails on mswin.
+assert_equal %q{[]}, %q{
+  Thread.new{sleep}.backtrace
+}

bootstraptest/test_thread.rb

@@ -19,9 +19,6 @@ assert_equal %q{ok}, %q{
     :ok
   }.value
 }
-assert_equal %q{[]}, %q{
-  Thread.new{sleep}.backtrace
-}
 assert_equal %q{ok}, %q{
 begin
   v = 0