test: removes string literals from test-domain-timer.js

String literals from two `assert.strictEquals()` calls were
removed as these string literals did not match the value of
`e.message` when an `AssertionError` occurred. Now the default
error message is displayed when an error occurs.

PR-URL: #20120
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-domain-timers.js

@@ -27,8 +27,7 @@ const assert = require('assert');
 const timeoutd = domain.create();
 
 timeoutd.on('error', common.mustCall(function(e) {
-  assert.strictEqual(e.message, 'Timeout UNREFd',
-                     'Domain should catch timer error');
+  assert.strictEqual(e.message, 'Timeout UNREFd');
   clearTimeout(timeout);
 }, 2));
 
@@ -47,8 +46,7 @@ t.unref();
 const immediated = domain.create();
 
 immediated.on('error', common.mustCall(function(e) {
-  assert.strictEqual(e.message, 'Immediate Error',
-                     'Domain should catch immediate error');
+  assert.strictEqual(e.message, 'Immediate Error');
 }));
 
 immediated.run(function() {