test: refactor test-tls-inception

* buffer-to-string comparison replaced with string-to-string comparison
* equal -> strictEqual
* var -> const
* rename identifiers to avoid masking

PR-URL: https://github.com/nodejs/node/pull/9536


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>