test: use `princ' instead of `message' calls in emacs tests

The patch replaces all (message (buffer-string)) calls in emacs
tests with (princ (buffer-string)).  This avoids accidentally
interpreting '%' as format specifiers and makes code simpler
because we do not need to capture stderr.

Also, the patch works around an Emacs (23.3+1-1 on current Debian
Unstable) segfault in "Ensure that emacs doesn't drop results"
test.  Note: the segfault does not happen on every test run.
Though, it seems to be consistently reproducible if the test uses
300 messages instead of 30.  Hopefully, it is the crash described
in Emacs bug #8545 [1] which is already fixed.

[1] http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8545