tools: enable no-self-assign ESLint rule

Enabled no-self-assign rule in ESLint.

This required one change in a benchmark file. Changed a loop (that is
outside of the benchmark itself, so performance is not critical) from a
for loop that repeats a string to use String.prototype.repeat() instead.

While at it, took the opportunity to const-ify the benchmark file.

Also moved the "Strict" section in the .eslintrc to match where it is in
the ESLint documentation. Updated the link for Strict rules to point to
the ESLint website rather than the GitHub-hosted code.

PR-URL: https://github.com/nodejs/node/pull/5552


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: targos - Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>