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/README-testing.c

@@ -0,0 +1,19 @@
+/* This file contains an example test case which shows minimal use of
+   the test framework.  Additional testing hooks are described in
+   <support/test-driver.c>.  */
+
+/* This function will be called from the test driver.  */
+static int
+do_test (void)
+{
+  if (3 == 5)
+    /* Indicate failure.  */
+    return 1;
+  else
+    /* Indicate success.  */
+    return 0;
+}
+
+/* This file references do_test above and contains the definition of
+   the main function.  */
+#include <support/test-driver.c>