fix: add stacktrace to failed expect function call

Author: alexandre-abrioux

Diff for: lib/test.js

@@ -303,7 +303,12 @@ function wrapAssertFn(assertFn) {
 
   return function(res) {
     let badStack;
-    const err = assertFn(res);
+    let err;
+    try {
+      err = assertFn(res);
+    } catch (e) {
+      err = e;
+    }
     if (err instanceof Error && err.stack) {
       badStack = err.stack.replace(err.message, '').split('\n').slice(1);
       err.stack = [err.toString()]