Add Nanos6 emu test to check strict nesting model

test/emu/nanos6/CMakeLists.txt

@@ -6,6 +6,7 @@ test_emu(nested-tasks-bad.c SHOULD_FAIL
   REGEX "body_execute: body(id=1,taskid=1) state must be Created but is Running")
 test_emu(task-types.c MP)
 test_emu(blocking.c MP)
+test_emu(if0-pause-parent.c)
 test_emu(ss-mismatch.c SHOULD_FAIL
   REGEX "thread [0-9]\\+ ended with 1 stacked nanos6 subsystems")
 test_emu(delayed-connect-ss.c)

test/emu/nanos6/if0-pause-parent.c

@@ -0,0 +1,46 @@
+/* Copyright (c) 2023 Barcelona Supercomputing Center (BSC)
+ * SPDX-License-Identifier: GPL-3.0-or-later */
+
+#include <stdint.h>
+#include "compat.h"
+#include "instr.h"
+#include "instr_nanos6.h"
+
+/* Ensure that we can accept a Nanos6 task model in which the parent gets paused
+ * before the if0 task begins. */
+
+int
+main(void)
+{
+	instr_start(0, 1);
+	instr_nanos6_init();
+
+	int ntasks = 100;
+	uint32_t typeid = 1;
+
+	instr_nanos6_type_create(typeid);
+
+	/* Create and run the tasks, one nested into another */
+	for (int32_t id = 1; id <= ntasks; id++) {
+		instr_nanos6_handle_task_enter();
+		instr_nanos6_task_create_begin();
+		instr_nanos6_task_create(id, typeid);
+		instr_nanos6_task_create_end();
+		instr_nanos6_task_body_enter();
+		instr_nanos6_task_execute(id);
+		sleep_us(500);
+		instr_nanos6_task_pause(id);
+	}
+
+	/* End the tasks in the opposite order */
+	for (int32_t id = ntasks; id >= 1; id--) {
+		instr_nanos6_task_resume(id);
+		instr_nanos6_task_end(id);
+		instr_nanos6_task_body_exit();
+		instr_nanos6_handle_task_exit();
+	}
+
+	instr_end();
+
+	return 0;
+}