Harden the ObjectSpace.memsize_of spec

[Bug #20803]

`abc` is used a lot across the ruby spec suite, if another test runs before
this spec is loaded and create this symbol dynamically (`"abc".to_sym`)
the spec will fail.

So it's preferable to use a symbol name that is very unlikely to be
used elsewhere to avoid flakes.

spec/ruby/library/objectspace/memsize_of_spec.rb

@@ -13,7 +13,7 @@ describe "ObjectSpace.memsize_of" do
   end
 
   it "returns 0 for literal Symbols" do
-    ObjectSpace.memsize_of(:abc).should == 0
+    ObjectSpace.memsize_of(:object_space_memsize_spec_static_sym).should == 0
   end
 
   it "returns a positive Integer for an Object" do