coin moves tests: save updates to disk

If we don't save to disk, if the node restarts we'll lose them all
and the resulting balance check at the end will be incorrect.

tests/plugins/coin_movements.py

@@ -2,12 +2,22 @@
 
 from pyln.client import Plugin
 
+import json
+import os.path
+
+
 plugin = Plugin()
 
 
 @plugin.init()
 def init(configuration, options, plugin):
-    plugin.coin_moves = []
+    if os.path.exists('moves.json'):
+        jd = {}
+        with open('moves.json', 'r') as f:
+            jd = f.read()
+        plugin.coin_moves = json.loads(jd)
+    else:
+        plugin.coin_moves = []
 
 
 @plugin.subscribe("coin_movement")
@@ -30,10 +40,21 @@ def notify_coin_movement(plugin, coin_movement, **kwargs):
 
     plugin.coin_moves.append(coin_movement)
 
+    # we save to disk so that we don't get borked if the node restarts
+    # assumes notification calls are synchronous (not thread safe)
+    with open('moves.json', 'w') as f:
+        f.write(json.dumps(plugin.coin_moves))
+
 
 @plugin.method('listcoinmoves_plugin')
 def return_moves(plugin):
-    return {'coin_moves': plugin.coin_moves}
+    result = []
+    if os.path.exists('moves.json'):
+        jd = {}
+        with open('moves.json', 'r') as f:
+            jd = f.read()
+        result = json.loads(jd)
+    return {'coin_moves': result}
 
 
 plugin.run()