sadt: Split Tests, Features, Restrictions according to the spec

autopkgtest documents these as being separated by "commas and/or whitespace", implying that having tests separated only by commas (no whitespace!) is valid. sadt previously required at least one whitespace character, and otherwise would treat commas as part of the test name.

Similarly, autopkgtest accepts and ignores a trailing comma, but sadt previously treated it as part of the name of the last test. This was particularly inconvenient when using debputy reformat or wrap-and-sort -t, which always emit a trailing comma.

Instead of parsing using regular expressions, use the same string operations as the reference implementation, autopkgtest.

Closes: #1121898