Adds a `-j`/`--workers-count` parameter to `tools/packaging/test262.py`, defaulting to `[number of cores] - 1`.

Speeds up running the test suite by about ~3x on my 4-core machine, with the SpiderMonkey shell. This could certainly be optimized more by just appending test results to per-thread lists and merging them at the end, but it's better than nothing.

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