testsuite: Use printk for all test output where possible.

- Remove the printf support leaving the direct printk support configured
  with TESTS_USE_PRINTK and all other output goes via a buffered vsniprintf
  call to printk.
- Control the test's single init for functions and global data with
  TEST_INIT and not CONFIGURE_INIT. They are now separate.

Updates #3170.

testsuites/libtests/termios07/init.c

@@ -11,6 +11,8 @@
 #include "config.h"
 #endif
 
+#define TEST_INIT
+
 #include <tmacros.h>
 #include "test_support.h"
 #include "termios_testdriver_intr.h"