Add unit test for the directives formatter function for PBS handler.

Also upgrade default PBS job name line length from 15 to 236, i.e. assuming PBS version >=13.

This is a small change with no associated Issue. The purpose is to observe the test coverage of a source file that is currently untested on Travis CI.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).

• Appropriate tests are included (unit and/or functional).

• Appropriate change log entry included.

• (master branch) I have opened a documentation PR at Update PBS job name max len from 15 to 236 cylc-doc#79.