It was recently decided to prefer the new `id` tag over the existing
`es5id` and `es6id` tag when authoring tests. Update the contribution
guidelines to reference the new tag.

Fixes #477

Some tests involving the directive prologue are invalidated by source
text transformations that insert executable code in the beginning of the
script. Implement a `raw` flag that allows these tests to opt-out of
this transformation. Update the relevant tests to use this flag (and
remove references to globals only available when code is injected).

Update the Python runner accordingly:

- Do not run tests marked as "raw" in strict mode
- Reject invalid test configurations

Update the browser runner accordingly:

- Do not modify the script body of tests marked as "raw"

Unlike the console runner, the browser runner does not modify the
strictness of tests prior to running them. Regardless of a given test's
metadata, it runs every test exactly once, and it never enables strict
mode. This means that tests intended to function in strict mode must
declare the "use strict"; directive prologue in addition to the
`onlyStrict` flag.

For any test that specifies the `onlyStrict` metadata flag, transform
the source code by injecting a "use strict" directive prologue prior to
running the test.

Introduce the `module` flag to unambiguously identify tests that are
intended to be interpreted as module code.

- Remove trailing white space
- Streamline documentation of test tags
- Do not reference obsolete tags
- Document `features` frontmatter tag
- Document `es6id` frontmatter tag
- Omit unnecessary detail about test262 website generation. This is not
  directly useful to potential test contributors. Implementation details
  like these can be taken for granted by that audience.
- Remove documentation on YAML syntax. Details on YAML may be helpful
  for some new contributors, but this document should not attempt to
  cover the topic (especially not from the description of a specific
  frontmatter entry). Replace with a link to a more comprehensive source
  as this will be more generally useful to those who need it (and less
  obtrusive for those who do not).
- Consolidate information on test helpers
- Document `assert` helpers
- Update instructions for asserting errors. Since the introduction of
  `assert.throws` in gh-22, the preferred means of expressing
  expectations regarding errors has changed. Update the CONTRIBUTING.md
  file to reflect the latest approach. Explain purpose of `throw
  NotEarlyError;` in example test.
- Re-order information on file names. The inconsistency in the project's
  file names should not go unmentioned, but neither should it not
  preceed instructions for the accepted approach to namine tests.
- More clearly document required frontmatter tags. Explicitly list
  `description` as a required frontmatter tag, implicitly identifying
  all other tags as optional.

CONTRIBUTING.md
- document `timeout` tag
- reorder tags in frontmatter doc
- minor cleanups
- minor fixes
- add style note
- reformat flags
- remove discussion of obsolete $INCLUDE
- incorporate line notes from @domenic
- integrate additional comments
- add links back, move arg notes down
- Raise outline level by one

README.md
- link to CONTRIBUTING

Minor fixes: typos, formatting, verb tense&agreement
Add example of short-list `includes: []` and long-list `includes: \n - \n -`

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

add a table showing which print handle to use when
running async tests through test262.py runner

@anba contributed information about async tests in
SpiderMonkey and JavaScriptCore

add a section on requirements for using console test runner test262.py

add a section on asynchronous tests

add subsection to authoring guidelines with suggestion for test names