test: cleanup test-child-process-exec-env.js

Replace equal with strictEqual, use const instead of var and
improve test with use of assert.notStrictEqual

PR-URL: https://github.com/nodejs/node/pull/8600


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>