Marked a ThreadLocalVar test pending on Rbx

The test passes on all platforms except Rbx-2 where it consistently
fails. We believe the problem is with the call to GC.start within the
test. As far as we know the code under test *should* work on all
platforms because it is pure Ruby. We cannot say for certain at this
time.

Author: jdantonio

spec/concurrent/atomic/thread_local_var_spec.rb

@@ -51,20 +51,20 @@ module Concurrent
           expect(var.instance_variable_get(:@storage).keys.size).to be == 1
         end
 
-        unless rbx?
-          #NOTE: This test depends on GC which works differently under Rbx
-          it 'does not leave values behind when bind is not used' do
-            tries = Array.new(10) do
-              var = ThreadLocalVar.new(0)
-              10.times.map do |i|
-                Thread.new { var.value = i; var.value }
-              end.each(&:join)
-              var.value = 0
-              GC.start
-              var.instance_variable_get(:@storage).keys.size == 1
-            end
-            expect(tries.any?).to be_truthy
+        it 'does not leave values behind when bind is not used' do
+          if rbx?
+            pending('fails on Rbx, possibly due to test dependency on GC')
           end
+          tries = Array.new(10) do
+            var = ThreadLocalVar.new(0)
+            10.times.map do |i|
+              Thread.new { var.value = i; var.value }
+            end.each(&:join)
+            var.value = 0
+            GC.start
+            var.instance_variable_get(:@storage).keys.size == 1
+          end
+          expect(tries.any?).to be_truthy
         end
       end
     end