test: allow user to choose which emacs to run tests with.

As we start to pay more attention to emacs24, it helps to be able to
select a different version of emacs to run the tests with to verify
version specific bugs.

A separate variable TEST_EMACS is needed to avoid being overwritten by the
make variable EMACS in Makefile.config

For what it's worth, the value of emacs is chosen at the time
tmp.emacs/run_emacs is created, so is fixed for all subtests.