lint: Simplify no-warnings tests.

* tests/lint.scm ("description: may start with a digit"): Replace
  (not (string-contains exp str)) with (string-null? exp).
  ("description: may start with lower-case package name"): Likewise.
  ("description: end-of-sentence detection with abbreviations"):
  Likewise.
  ("synopsis: may start with a digit"): Likewise.
  ("synopsis: ends with 'etc.'"): Likewise.