test: add test-debug-protocol-execute

Add test for `Protocol` object in `_debugger` module. This test covers
some edge cases that fill some coverage gaps in our testing (such as the
"Unknown state" error).

PR-URL: nodejs#8454
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-debug-protocol-execute.js

@@ -0,0 +1,20 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const debug = require('_debugger');
+
+const protocol = new debug.Protocol();
+
+assert.strictEqual(protocol.state, 'headers');
+
+protocol.execute('Content-Length: 10\r\n\r\nfhqwhgads');
+
+assert.strictEqual(protocol.state, 'body');
+assert.strictEqual(protocol.res.body, undefined);
+
+protocol.state = 'sterrance';
+assert.throws(
+	() => { protocol.execute('grumblecakes'); },
+	/^Error: Unknown state$/
+);