Update test/* directory, it seem to be a bit bit-rotted

Added README; modified sigkill_rain.c to be more understandable,
made clone.c compile; added wait_must_be_interruptible.c test;
updated Makefile and .gitignore.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/test/clone.c b/test/clone.c
index 335086f..5de302f 100644
--- a/test/clone.c
+++ b/test/clone.c
@@ -1,3 +1,6 @@
+/* for CLONE_foo: */
+#define _GNU_SOURCE 1
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <sched.h>