support: Introduce new subdirectory for test infrastructure

The new test driver in <support/test-driver.c> has feature parity with
the old one.  The main difference is that its hooking mechanism is
based on functions and function pointers instead of macros.  This
commit also implements a new environment variable, TEST_COREDUMPS,
which disables the code which disables coredumps (that is, it enables
them if the invocation environment has not disabled them).

<test-skeleton.c> defines wrapper functions so that it is possible to
use existing macros with the new-style hook functionality.

This commit changes only a few test cases to the new test driver, to
make sure that it works as expected.

support/xasprintf.c

@@ -0,0 +1,38 @@
+/* Error-checking wrapper for asprintf.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/support.h>
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+char *
+xasprintf (const char *format, ...)
+{
+  va_list ap;
+  va_start (ap, format);
+  char *result;
+  if (vasprintf (&result, format, ap) < 0)
+    {
+      printf ("error: asprintf: %m\n");
+      exit (1);
+    }
+  va_end (ap);
+  return result;
+}