pytest: test passing utf8 into and out of plugin JSON.

We should be able to pass UTF-8 strings to and from plugins without
python turning them into JSON-\u escapes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/utf8.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python3
+from lightning import Plugin
+
+
+plugin = Plugin()
+
+
+@plugin.method("utf8")
+def echo(plugin, utf8):
+    assert '\\u' not in utf8
+    return {'utf8': utf8}
+
+
+plugin.run()