build tests: do ldc2 tests even if omit_gdc_tests set

spec/build_tests_spec.rb

@@ -591,7 +591,6 @@ EOF
     expect(result.stdout).to eq ""
   end
 
-  unless ENV["omit_gdc_tests"]
   it "supports building D sources with gdc" do
     test_dir("d")
     result = run_rscons
@@ -636,6 +635,7 @@ EOF
     expect(`./hello-d.exe`.rstrip).to eq "Hello from D, value is 33!"
   end
 
+  unless ENV["omit_gdc_tests"]
     it "links with the D linker when object files were built from D sources" do
       test_dir("d")
       result = run_rscons(rsconscript: "link_objects.rb")