qa: introduce runners and more tests for newlib

For each test, if a script exists with the same name of the test
plus the .runner suffix, the runner is run instead of the test.

As a first example qa/lib/newlib/testsuite/atexit is run by
qa/lib/newlib/testsuite/atexit.runner.

These .runner scripts allow more complex checks of the side effects
generated by the test.

qa/lib/newlib/000-hello.c

@@ -0,0 +1,7 @@
+/* let's start with a K&R classic */
+#include <stdio.h>
+int main()
+{
+   printf("hello, world\n");
+   return 0;
+}