Add test case for error stringifier

src/m2d/event-dispatcher.test.js

@@ -0,0 +1,23 @@
+// @ts-check
+
+const {test} = require("supertape")
+const {stringifyErrorStack} = require("./event-dispatcher")
+
+test("stringify error stack: works", t => {
+	function a() {
+		const e = new Error("message", {cause: new Error("inner")})
+		// @ts-ignore
+		e.prop = 2.1
+		throw e
+	}
+	try {
+		a()
+		t.fail("shouldn't get here")
+	} catch (e) {
+		const str = stringifyErrorStack(e)
+		t.match(str, /^Error: message$/m)
+		t.match(str, /^    at a \(.*event-dispatcher\.test\.js/m)
+		t.match(str, /^  \[cause\]: Error: inner$/m)
+		t.match(str, /^  \[prop\]: 2.1$/m)
+	}
+})