magit-tests.el: remove magit-blame-mode test

There are hundreds of commands, why test just this particular one?
magit-tests--modify-file: cosmetics
magit-tests--should-have-section: new function

Add new function `magit-tests--should-have-section' replacing
`magit-tests--should-have-item-title'.  This is mostly a semantic
cleanup and doesn't change the fact that this function is very
limited and will likely have to be replaced again when we add
new tests.