test-lib-functions.sh: use '%s\n' instead of "%s\n"

test-lib-functions.sh: use '%s\n' instead of "%s\n"

Using "%s\n" relies on one of two things happening:

  1) The '\n' part passes through unmolested (i.e. the '\' is NOT removed)
  2) The '\n' is replaced with an actual literal newline

Technically the POSIX standard requires (1) since the backslash is not
followed by '$', '`', '"', '\' or newline, but everywhere else the same
string is placed in single quotes (e.g. '%s\n') instead.  Double escaping
the backslash (i.e. "%s\\n") would also work but is less pretty and not
consistent with the other uses.

Therefore change out this one "%s\n" to use '%s\n' instead to be clearer,
more consistent and not rely on a bare "\n" remaining unmolested.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>