TestTime#test_memsize: skip when on GC_DEBUG

GC_DEBUG=1 makes this test fail because it changes the size of struct
RVALUE.  I don't think the test is useful then.  Let's just skip.

gc.c

@@ -11851,6 +11851,7 @@ Init_GC(void)
     rb_mGC = rb_define_module("GC");
 
     gc_constants = rb_hash_new();
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("DEBUG")), GC_DEBUG ? Qtrue : Qfalse);
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_SIZE")), SIZET2NUM(sizeof(RVALUE)));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_PAGE_OBJ_LIMIT")), SIZET2NUM(HEAP_PAGE_OBJ_LIMIT));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_PAGE_BITMAP_SIZE")), SIZET2NUM(HEAP_PAGE_BITMAP_SIZE));

test/ruby/test_time.rb

@@ -1256,6 +1256,7 @@ class TestTime < Test::Unit::TestCase
   def test_memsize
     # Time objects are common in some code, try to keep them small
     skip "Time object size test" if /^(?:i.?86|x86_64)-linux/ !~ RUBY_PLATFORM
+    skip "GC is in debug" if GC::INTERNAL_CONSTANTS[:DEBUG]
     require 'objspace'
     t = Time.at(0)
     size = GC::INTERNAL_CONSTANTS[:RVALUE_SIZE]