tests/extmod/btree1: Tests against in-memory DB (using io.BytesIO).

tests/extmod/btree1.py

@@ -1,11 +1,15 @@
 try:
     import btree
+    import uio
 except ImportError:
     print("SKIP")
     import sys
     sys.exit()
 
-db = btree.open(None)
+#f = open("_test.db", "w+b")
+f = uio.BytesIO()
+db = btree.open(f)
+
 db[b"foo3"] = b"bar3"
 db[b"foo1"] = b"bar1"
 db[b"foo2"] = b"bar2"
@@ -59,3 +63,4 @@ for k in db:
     print(k)
 
 db.close()
+f.close()