[gdb] Add unit test for gdb::sequential_for_each

With commit 18a5766 ("[gdbsupport] Add sequential_for_each") I added a
drop-in replacement for gdb::parallel_for_each, but there's nothing making
sure that the two remain in sync.

Extend the unit test for gdb::parallel_for_each to test both.

Do this using a slightly unusual file-self-inclusion.  Doing so keep things
readable and maintainable, and avoids macrofying functions.

Tested on x86_64-linux.

gdb/unittests/parallel-for-selftests.c

@@ -17,6 +17,13 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+/* This file is divided in two parts:
+   - FOR_EACH-undefined, and
+   - FOR_EACH-defined.
+   The former includes the latter, more than once, with different values for
+   FOR_EACH.  The FOR_EACH-defined part reads like a regular function.  */
+#ifndef FOR_EACH
+
 #include "defs.h"
 #include "gdbsupport/selftest.h"
 #include "gdbsupport/parallel-for.h"
@@ -43,29 +50,74 @@ struct save_restore_n_threads
   int n_threads;
 };
 
+/* Define test_par using TEST in the FOR_EACH-defined part.  */
+#define TEST test_par
+#define FOR_EACH gdb::parallel_for_each
+#include "parallel-for-selftests.c"
+#undef FOR_EACH
+#undef TEST
+
+/* Define test_seq using TEST in the FOR_EACH-defined part.  */
+#define TEST test_seq
+#define FOR_EACH gdb::sequential_for_each
+#include "parallel-for-selftests.c"
+#undef FOR_EACH
+#undef TEST
+
 static void
 test (int n_threads)
+{
+  test_par (n_threads);
+  test_seq (n_threads);
+}
+
+static void
+test_n_threads ()
+{
+  test (0);
+  test (1);
+  test (3);
+}
+
+}
+}
+
+#endif /* CXX_STD_THREAD */
+
+void _initialize_parallel_for_selftests ();
+void
+_initialize_parallel_for_selftests ()
+{
+#ifdef CXX_STD_THREAD
+  selftests::register_test ("parallel_for",
+			    selftests::parallel_for::test_n_threads);
+#endif /* CXX_STD_THREAD */
+}
+
+#else /* FOR_EACH */
+
+static void
+TEST (int n_threads)
 {
   save_restore_n_threads saver;
   gdb::thread_pool::g_thread_pool->set_thread_count (n_threads);
 
 #define NUMBER 10000
 
   std::atomic<int> counter (0);
-  gdb::parallel_for_each (1, 0, NUMBER,
-			  [&] (int start, int end)
-			  {
-			    counter += end - start;
-			  });
-
+  FOR_EACH (1, 0, NUMBER,
+	    [&] (int start, int end)
+	    {
+	      counter += end - start;
+	    });
   SELF_CHECK (counter == NUMBER);
 
   counter = 0;
-  gdb::parallel_for_each (1, 0, 0,
-			  [&] (int start, int end)
-			  {
-			    counter += end - start;
-			  });
+  FOR_EACH (1, 0, 0,
+	    [&] (int start, int end)
+	    {
+	      counter += end - start;
+	    });
   SELF_CHECK (counter == 0);
 
   auto task_size_max_ = [] (int iter)
@@ -75,11 +127,11 @@ test (int n_threads)
   auto task_size_max = gdb::make_function_view (task_size_max_);
 
   counter = 0;
-  gdb::parallel_for_each (1, 0, NUMBER,
-			  [&] (int start, int end)
-			  {
-			    counter += end - start;
-			  }, task_size_max);
+  FOR_EACH (1, 0, NUMBER,
+	    [&] (int start, int end)
+	    {
+	      counter += end - start;
+	    }, task_size_max);
   SELF_CHECK (counter == NUMBER);
 
   auto task_size_one_ = [] (int iter)
@@ -89,35 +141,14 @@ test (int n_threads)
   auto task_size_one = gdb::make_function_view (task_size_one_);
 
   counter = 0;
-  gdb::parallel_for_each (1, 0, NUMBER,
-			  [&] (int start, int end)
-			  {
-			    counter += end - start;
-			  }, task_size_one);
+  FOR_EACH (1, 0, NUMBER,
+	    [&] (int start, int end)
+	    {
+	      counter += end - start;
+	    }, task_size_one);
   SELF_CHECK (counter == NUMBER);
 
 #undef NUMBER
 }
 
-static void
-test_n_threads ()
-{
-  test (0);
-  test (1);
-  test (3);
-}
-
-}
-}
-
-#endif /* CXX_STD_THREAD */
-
-void _initialize_parallel_for_selftests ();
-void
-_initialize_parallel_for_selftests ()
-{
-#ifdef CXX_STD_THREAD
-  selftests::register_test ("parallel_for",
-			    selftests::parallel_for::test_n_threads);
-#endif /* CXX_STD_THREAD */
-}
+#endif /* FOR_EACH */