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