test: emacs: fix a couple of shellcheck complaints

test: emacs: fix a couple of shellcheck complaints

In test-lib-emacs.sh line 20:
    test_require_external_prereq ${TEST_EMACS} || ret=1
                                 ^-----------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
    test_require_external_prereq "${TEST_EMACS}" || ret=1

In test-lib-emacs.sh line 21:
    test_require_external_prereq ${TEST_EMACSCLIENT} || ret=1
                                 ^-----------------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
    test_require_external_prereq "${TEST_EMACSCLIENT}" || ret=1

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>