Explicitly quit gdb from DAP server thread

This changes the DAP code to explicitly request that gdb exit.
Previously this could cause crashes, but with the previous cleanups,
this should no longer happen.

This also adds a tests that ensures that gdb exits with status 0.

gdb/python/lib/gdb/dap/server.py

@@ -230,6 +230,7 @@ class Server:
         # responses are flushed to the client before exiting.
         self.write_queue.put(None)
         json_writer.join()
+        send_gdb("quit")
 
     @in_dap_thread
     def send_event_later(self, event, body=None):

gdb/testsuite/lib/dap-support.exp

@@ -403,6 +403,15 @@ proc dap_check_log_file_re { re } {
 proc dap_shutdown {{terminate false}} {
     dap_check_request_and_response "shutdown" disconnect \
 	[format {o terminateDebuggee [l %s]} $terminate]
+
+    # Check gdb's exit status.
+    global gdb_spawn_id
+    set result [wait -i $gdb_spawn_id]
+    gdb_assert {[lindex $result 2] == 0}
+    gdb_assert {[lindex $result 3] == 0}
+
+    clear_gdb_spawn_id
+
     dap_check_log_file
 }