test: remove the third string literal argument from assert.strictEqual()

Third string literal argument in assert.strictEqual() needs to be
removed. Otherwise, on AssertionError it would not display the
values that failed the check -- this hinders debugging.
The string literals are added as comments above the check.

PR-URL: #21406
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-string-decoder-end.js

@@ -103,8 +103,10 @@ function testBuf(encoding, buf) {
   // .toString() on the buffer
   const res3 = buf.toString(encoding);
 
-  assert.strictEqual(res1, res3, 'one byte at a time should match toString');
-  assert.strictEqual(res2, res3, 'all bytes at once should match toString');
+  // One byte at a time should match toString
+  assert.strictEqual(res1, res3);
+  // All bytes at once should match toString
+  assert.strictEqual(res2, res3);
 }
 
 function testEnd(encoding, incomplete, next, expected) {