Fix failing tests on Linux

GCC does not accept an extern "C" storage class specifier along with
definition of the global variable. Remove the specifier and move to the
global namespace to fix this.

tests/ffi/tests.cc

@@ -14,6 +14,8 @@ extern "C" void cxx_test_suite_set_correct() noexcept;
 extern "C" tests::R *cxx_test_suite_get_box() noexcept;
 extern "C" bool cxx_test_suite_r_is_correct(const tests::R *) noexcept;
 
+bool PmrDeleterForC_callback_used{false};
+
 namespace tests {
 
 #if __cplusplus < 201703L
@@ -139,8 +141,6 @@ std::unique_ptr<C> c_return_unique_ptr() {
   return std::unique_ptr<C>(new C{2020});
 }
 
-extern "C" bool PmrDeleterForC_callback_used{false};
-
 void PmrDeleterForC::operator()(C* obj) {
     alloc.destroy(obj);
     alloc.deallocate(obj, 1);