fix: improve test coverage for completions

test/completion.cjs

@@ -359,8 +359,28 @@ describe('Completion', () => {
           return done();
         }
       );
-    })
+    });
 
+    it('allows calling callback instead of default completion function', done => {
+      checkUsage(
+        () => {
+          yargs(['./completion', '--get-yargs-completions'])
+            .command('foo', 'bar')
+            .command('apple', 'banana')
+            .completion('completion', (current, argv, defaultCompletion, done) => {
+	      done(['orange'])
+            })
+            .parse();
+        },
+        null,
+        (err, r) => {
+          if (err) throw err;
+          r.logs.should.include('orange');
+          r.logs.should.not.include('foo');
+          return done();
+        }
+      );
+    });
 
     it('if a promise is returned, completions can be asynchronous', done => {
       checkUsage(