tests: Solaris sed does not support "\n" meaning newline in the RHS of s///
The blackbox test rewrites a copy of test-dispatch.py on the fly, and adds
a couple of lines with the s/// command. GNU sed supports the use of the
\n escape to represent a newline, but not Solaris sed. Using a literal
newline, prefixed by a backslash, works with both versions of the utility.