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/sigkill_rain.c b/test/sigkill_rain.c
index dd83a69..01b8986 100644
--- a/test/sigkill_rain.c
+++ b/test/sigkill_rain.c
@@ -1,12 +1,10 @@
-/* This test is not yet added to Makefile */
-
 #include <stdlib.h>
 #include <stddef.h>
 #include <unistd.h>
 #include <signal.h>
-
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <stdio.h>
 
 static const struct sockaddr sa;
 
@@ -16,6 +14,12 @@
 	int pid;
 	sigset_t set;
 
+	printf(
+"Please run me under 'strace -f -oLOG', and examine LOG file for incorrect\n"
+"decoding of interrupted syscalls: grep for 'sendto', '??" /* anti-trigraph gap */ "?', 'unavailable'.\n"
+"Pass number of iterations in argv[1] (default: 999).\n"
+	);
+
 	sigemptyset(&set);
 	sigaddset(&set, SIGCHLD);
 	sigprocmask(SIG_BLOCK, &set, NULL);
@@ -57,7 +61,7 @@
 				 */
 				switch (loops & 1) {
 				case 0:
-					break; /* intentional empty */
+					break; /* intentionally empty */
 				case 1:
 					sendto(-1, "Hello cruel world", 17, 0, &sa, sizeof(sa));
 					break;