Fix guards in specs using C extension classes

* Fixes #1080

Author: eregon

spec/concurrent/atomic/atomic_boolean_spec.rb

@@ -142,7 +142,7 @@ module Concurrent
     end
   end
 
-  if Concurrent.allow_c_extensions?
+  if Concurrent.c_extensions_loaded?
     RSpec.describe CAtomicBoolean do
       it_should_behave_like :atomic_boolean
     end
@@ -165,7 +165,7 @@ module Concurrent
       it 'inherits from JavaAtomicBoolean' do
         expect(AtomicBoolean.ancestors).to include(JavaAtomicBoolean)
       end
-    elsif Concurrent.allow_c_extensions?
+    elsif Concurrent.c_extensions_loaded?
       it 'inherits from CAtomicBoolean' do
         expect(AtomicBoolean.ancestors).to include(CAtomicBoolean)
       end

spec/concurrent/atomic/atomic_fixnum_spec.rb

@@ -204,7 +204,7 @@ module Concurrent
     end
   end
 
-  if Concurrent.allow_c_extensions?
+  if Concurrent.c_extensions_loaded?
     RSpec.describe CAtomicFixnum do
       it_should_behave_like :atomic_fixnum
     end
@@ -227,7 +227,7 @@ module Concurrent
       it 'inherits from JavaAtomicFixnum' do
         expect(AtomicFixnum.ancestors).to include(JavaAtomicFixnum)
       end
-    elsif Concurrent.allow_c_extensions?
+    elsif Concurrent.c_extensions_loaded?
       it 'inherits from CAtomicFixnum' do
         expect(AtomicFixnum.ancestors).to include(CAtomicFixnum)
       end

spec/concurrent/atomic/atomic_reference_spec.rb

@@ -163,7 +163,7 @@ module Concurrent
     it_should_behave_like :atomic_reference
   end
 
-  if Concurrent.allow_c_extensions?
+  if Concurrent.c_extensions_loaded?
     RSpec.describe CAtomicReference do
       it_should_behave_like :atomic_reference
     end
@@ -190,7 +190,7 @@ module Concurrent
       it 'inherits from JavaAtomicReference' do
         expect(described_class.ancestors).to include(Concurrent::JavaAtomicReference)
       end
-    elsif Concurrent.allow_c_extensions?
+    elsif Concurrent.c_extensions_loaded?
       it 'inherits from CAtomicReference' do
         expect(described_class.ancestors).to include(Concurrent::CAtomicReference)
       end