Converted atest/README and utest/README to reST.

Both actually were (mostly) valid reST already so the biggest change
was renaming files to use `.rst` extension. The commit also contains
some fixes to syntax and small enhancements to docs.
atests: New functionality to verify expected errors using glob pattern.

Now expected error can be specified using `FAIL GLOB:` in addition to
`FAIL REGEXP:` and `FAIL STARTS:`. The motivation was to ease matching
more complex or changing errors without needing to use regexps.
Glob patterns are simpler and, most importantly, require less escaping.

Pull request #1776 provides a concrete use case for this functionality.