tests: Use .errno instead of .args[0] for OSError exceptions.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/btree1.py

@@ -65,7 +65,7 @@ print(db.seq(1, b"qux"))
 try:
     db.seq(b"foo1")
 except OSError as e:
-    print(e.args[0] == uerrno.EINVAL)
+    print(e.errno == uerrno.EINVAL)
 
 print(list(db.keys()))
 print(list(db.values()))