YAML formatting restrictions

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 ]
• 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

No specific files or tests are named. Start by reviewing the Test262 test-file frontmatter importing processes and the YAML examples in this issue; identify the complete restriction list and define done as an agreed set of enforceable formatting rules.