Update tests to allow for non-default core.notesRef settings

...and finding a valid, but unexpected setting for core.notesRef on systems
where that setting has been configured probably doesn't constitute a test
failure :)

This is what I was seeing:

    t/24-note.t ................. 1/? 
    #   Failed test at t/24-note.t line 26.
    #          got: 'refs/notes/rsrchboy'
    #     expected: 'refs/notes/commits'
    # Looks like you failed 1 test of 17.

...as I have ```core.notesRef = refs/notes/rsrchboy``` in my ~/.gitconfig.

This PR simply changes to look up the value of ```core.notesRef``` and use
that in the test, rather than the hard-coded universal default ref it was
using previously.