testsuite: fix a spurious failure with non-bash shells

testsuite: fix a spurious failure with non-bash shells

* tests/tap-fancy2.test: Remove an unportable use of backslashes
with the `echo' builtin, which was causing some shells (among them
bash and Debian /bin/ksh) to print a `\\' string, while other
shells (among them zsh, dash, and Solaris /bin/sh and /bin/ksh)
were unexpectedly printing a single `\' character.  Since we are
at it, add a sanity check to ensure that this issue does not
resurface.