regress/r1test: Support a `$debug' prefix variable.

Amazingly, sometimes the tests don't pass.  When that happens, it's
handy to be able to debug the library and find out why.

It's a little tricky to attach GDB to the running process directly.  I
used the following:

	$ debug="noip gdbserver localhost:666" ./r1test CASE

and then, in another terminal, something like

	$ noip gdb adnstest_s
	(gdb) target remote localhost:666

to debug the program.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>