test: Add a new test_emacs function to test-lib.sh

test: Add a new test_emacs function to test-lib.sh

This should be quite handy for doing automated testing of the
emacs-based functionality in notmuch. This function invokes emacs with
the necessary command-line arguments, (to run in batch mode with no
local initialization, to load the notmuch code from the source
directory, and to ensure an 80-column width).