Specs for type validation of writes to $@

Use the shared specs for Exception#set_backtrace

Author: herwinw

language/predefined_spec.rb

@@ -1,4 +1,5 @@
 require_relative '../spec_helper'
+require_relative '../core/exception/shared/set_backtrace'
 require 'stringio'
 
 # The following tables are excerpted from Programming Ruby: The Pragmatic Programmer's Guide'
@@ -621,6 +622,17 @@ def foo
     end
   end
 
+  it_behaves_like :exception_set_backtrace, -> *backtrace {
+    exception = nil
+    begin
+      raise
+    rescue
+      $@ = backtrace.first unless backtrace.empty?
+      exception = $!
+    end
+    exception
+  }
+
   it "cannot be assigned when there is no a rescued exception" do
     -> {
       $@ = []