test: use template literals in test-string-decoder

`test/parallel/test-string-decoder.js` used to use string concatenation
this was migrated to use template literals. When concatenation
involved a newline we kept string concatenation, or to keep below 80
charters per line.

PR-URL: https://github.com/nodejs/node/pull/15884


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>