CONTRIBUTING: more notes on test writing

Add section on test environment
Add section on custom helpers
describe YAML frontmatter
Fix minor formatting errors
document $INCLUDE as obsolete
Change documentation of negative error
Move test environment and custom helpers down
indent copyright and frontmatter sections
better description of the async calls between a promise
and the functions in its `.then`
Correct Early Error example: don't throw a string

CONSOLE-RUNNER: split runner doc into new file
add troubleshooting section
es6/Math: use Number.isNaN

use number.isNaN to test for NaN
pointed out by @anba in #60
CONTRIBUTING.md: minor fixes

Minor fixes: typos, formatting, verb tense&agreement
Add example of short-list `includes: []` and long-list `includes: \n - \n -`
Merge pull request #59 from smikes/CONTRIBUTING-clarify

CONTRIBUTING: more notes on test writing
Merge pull request #66 from smikes/Math-isNaN

es6/Math: use Number.isNaN
bestPractice: supply error names to negative