tests: fix portability of sed usage in test-mq on some platform (Mac OS X and Solaris, at least), to insert new text line, sed function 'i' should be followed by: - backslash('\'), - new-line, - text to be inserted and - new-line GNU sed on Linux can recognize both previous and new ones as same modification request. in addition to it, this patch avoids to use '-i' option for sed, because it is not so portable, as noted in WritingTests wiki page.