test: Remove redundant "" arg to `mapconcat`

test: Remove redundant "" arg to `mapconcat`

* test/src/process-tests.el (process-test-stderr-filter):
* test/src/print-tests.el (print-tests-continuous-numbering):
* test/src/fns-tests.el (fns-tests-mapconcat):
* test/src/data-tests.el (test-bool-vector-to-hex-string):
* test/src/casefiddle-tests.el (casefiddle-tests-char-properties)
(casefiddle-tests-case-table, casefiddle-tests-casing-character):
* test/lisp/tabify-tests.el (tabify-tests--test-changes):
* test/lisp/subr-tests.el (subr-tests-bug22027):
* test/lisp/sort-tests.el (sort-tests-random-word):
* test/lisp/net/hmac-md5-tests.el (hmac-md5-test-encode-string):
* test/lisp/md4-tests.el (md4-tests-digest->hex):
* test/lisp/emacs-lisp/cl-extra-tests.el (cl-extra-test-map):
* test/lisp/dired-tests.el (dired-test-directory-files):
* test/lisp/char-fold-tests.el (char-fold--random-word):
* test/lisp/ansi-color-tests.el (ansi-color-incomplete-sequences-test):
Remove redundant "" arg to `mapconcat`.