test: add expectWarning to common

There are multiple tests that use the same boilerplate to test that
warnings are correctly emitted. This adds a new common function to do that
and changes the tests to use it.

PR-URL: #8662
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common.js

@@ -533,3 +533,17 @@ ArrayStream.prototype.writable = true;
 ArrayStream.prototype.pause = function() {};
 ArrayStream.prototype.resume = function() {};
 ArrayStream.prototype.write = function() {};
+
+exports.expectWarning = function(name, expected) {
+  if (typeof expected === 'string')
+    expected = [expected];
+  process.on('warning', exports.mustCall((warning) => {
+    assert.strictEqual(warning.name, name);
+    assert.ok(expected.includes(warning.message),
+              `unexpected error message: "${warning.message}"`);
+    // Remove a warning message after it is seen so that we guarantee that we
+    // get each message only once.
+    expected.splice(expected.indexOf(warning.message), 1);
+  }, expected.length));
+};
+