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.