test: improve coverage for 'internal/errors'

PR-URL: #16055
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: ruxandrafed

test/parallel/test-internal-errors.js

@@ -236,6 +236,15 @@ assert.throws(
     message: /^At least one arg needs to be specified$/
   }));
 
+// Test ERR_ENCODING_NOT_SUPPORTED
+assert.strictEqual(
+  errors.message('ERR_ENCODING_NOT_SUPPORTED', ['enc']),
+  'The "enc" encoding is not supported');
+
+// Test ERR_HTTP2_HEADER_REQUIRED
+assert.strictEqual(
+  errors.message('ERR_HTTP2_HEADER_REQUIRED', ['test']),
+  'The test header is required');
 
 // Test that `code` property is mutable and that changing it does not change the
 // name.