Disable a String#[] spec with big negative length on Windows

Author: andrykonchin

core/string/shared/slice.rb

@@ -122,7 +122,12 @@
   platform_is pointer_size: 64 do
     it "returns nil if the length is negative big value" do
       "hello there".send(@method, 4, -(1 << 31)).should == nil
-      "hello there".send(@method, 4, -(1 << 63)).should == nil
+
+      # by some reason length < -(1 << 31) on CI on Windows leads to
+      # 'RangeError: bignum too big to convert into `long'' error
+      platform_is_not :windows do
+        "hello there".send(@method, 4, -(1 << 63)).should == nil
+      end
     end
   end