test: improve _http_incoming.js coverage

PR-URL: #42211
Refs: https://coverage.nodejs.org/coverage-010cb714161102de/lib/_http_incoming.js.html
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-set-incoming-message-header.js

@@ -0,0 +1,27 @@
+'use strict';
+
+require('../common');
+const { IncomingMessage } = require('http');
+const assert = require('assert');
+
+// Headers setter function set a header correctly
+{
+  const im = new IncomingMessage();
+  im.headers = { key: 'value' };
+  assert.deepStrictEqual(im.headers, { key: 'value' });
+}
+
+// Trailers setter function set a header correctly
+{
+  const im = new IncomingMessage();
+  im.trailers = { key: 'value' };
+  assert.deepStrictEqual(im.trailers, { key: 'value' });
+}
+
+// _addHeaderLines function set a header correctly
+{
+  const im = new IncomingMessage();
+  im.headers = { key1: 'value1' };
+  im._addHeaderLines(['key2', 'value2'], 2);
+  assert.deepStrictEqual(im.headers, { key1: 'value1', key2: 'value2' });
+}