TESTS: add blocksig.c to run tests with all signals blocked

A problem was reported recently by some users of programs compiled
with Go 1.5 which by default blocks all signals before executing
processes, resulting in haproxy not receiving SIGUSR1 or even SIGTERM.

This program mimmicks this behaviour to make it easier to run tests.
It also displays the current signal mask. A simple test consists in
running it through itself.

tests/blocksig.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+#include <signal.h>
+#include <unistd.h>
+
+int main(int argc, char **argv)
+{
+	sigset_t new_sig, old_sig;
+
+	sigfillset(&new_sig);
+	sigprocmask(SIG_SETMASK, &new_sig, &old_sig);
+	printf("old_sig: %16Lx\n", *(unsigned long long*)&old_sig);
+	printf("new_sig: %16Lx\n", *(unsigned long long*)&new_sig);
+	
+	argc--; argv++;
+	return argc ? execvp(*argv, argv) : 0;
+}