test-skeleton.c: Use stdout for error messages

At the moment the test skeleton uses a mixture of stdout and
stderr for error message output. Using stdout for all test output
keeps all output correctly ordered and properly redirected to the
output file. The suggestion to use stdout is also made on the wiki:

https://sourceware.org/glibc/wiki/Testing/Testsuite#Writing_a_test_case

ChangeLog:

2014-06-23  Will Newton  <will.newton@linaro.org>

	* test-skeleton.c (signal_handler): Use printf and %m
	rather than perror.  Use printf rather than fprintf to
	stderr.  Use puts rather than fputs to stderr.
	(main): Likewise.