modules: Include config.h in all test files

Problem: By definition, all tests in the src/modules directory
are through non-public interfaces, so all test files should include
config.h.  However, several did not.

Add config.h to files that were missing them.

Author: chu11

src/modules/job-exec/test/bulk-exec.c

@@ -12,7 +12,9 @@
  *   execution API.
  */
 
-#define _GNU_SOURCE 1
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
 #include <unistd.h>
 #include <assert.h>
 #include <signal.h>

src/modules/job-exec/test/rset.c

@@ -8,6 +8,9 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
 #include <errno.h>
 #include <string.h>
 

src/modules/kvs/test/waitqueue.c

@@ -8,6 +8,10 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
+
 #include "src/modules/kvs/waitqueue.h"
 #include "src/common/libflux/message.h"
 #include "src/common/libtap/tap.h"