Don't make parser tests installable

This makes the regular build of odoc-parser to depend on its
test-dependencies, making it fail.

src/parser/test/dune

···
 (library
  (name odoc_parser_test)
- (package odoc-parser)
  (inline_tests)
  (enabled_if
   (>= %{ocaml_version} 4.04.1))