YAML formatting restrictions

[leobalter]

We should restrict the YAML format for the test files frontmatter to improve interoperability.

Many test runners depend on quick but limited yaml parsers and it's ok to have style restrictions identified in the importing processes for Test262.

Some of them I can already tell from scratch:

• [ ] no leading or trailing spaces from inline lists. e.g.: [ leading-space, foo, trailing-space ]
• [x] no leading or trailing blank lines from text blocks. e.g. the info tag cannot start with an empty line
• [ ] lists should always be inline. Always wrapped with square brackets and comma-space divided words. e.g.:
• [ ] no leading or trailing spaces for the block comment limiters. (See #2018)

features:
-  - var
-  - foo
-  - bar
+  [var, foo, bar]

There are might be more items we want to identify here. We should increase the list as necessary.

cc @caitp @gsathya @psmarshall

[gsathya]

Looks like JSC's parser is having trouble with the YAML as well: https://bugs.webkit.org/show_bug.cgi?id=193053

[leobalter]

@gsathya yes, the issues we can list here might affect at least V8, JSC and SpiderMonkey. Turns out it Yaml parsers are more challenging than one can expect.

---

Edit: For clarification, my goal is to tackle these issues and provide a consistent format to avoid any problem when consuming test262 in other projects.

[mathiasbynens]

Previously: #1817, #1821, #1868.

[gsathya]

Another one: https://github.com/tc39/test262/issues/2034

[littledan]

Thanks for working on this important issue, @leobalter.

[kmiller68]

Would it also be possible to prohibit inline value strings that span multiple lines? That seems to cause issues for JSC's YAML parser...

For example:

description: Security Throws a TypeError if start coercion detaches
  the buffer

Ideally, this would be formatted as one line or with "|"/">"

[rkirsling]

Opened #2505 to address the specific issue @kmiller68 identified.