signal: Avoid system signal disposition to interfere with tests

Both tst-sigset2 and tst-signal1 expectes that SIGINT disposition
is set to SIG_DFL.

signal/tst-sigset2.c

@@ -11,6 +11,8 @@
 #include <sys/wait.h>
 #include <libc-diag.h>
 
+#include <support/xsignal.h>
+
 /* The sigset function is deprecated.  */
 DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wdeprecated-declarations");
 
@@ -148,6 +150,8 @@ returnTest3 (void)
 int
 main (int argc, char *argv[])
 {
+  xsignal (TEST_SIG, SIG_DFL);
+
   pid_t childPid;
 
   childPid = fork();

sysdeps/pthread/tst-signal1.c

@@ -26,6 +26,7 @@
 #include <sys/wait.h>
 
 #include <support/xunistd.h>
+#include <support/xsignal.h>
 
 static sigset_t ss;
 static pthread_barrier_t *b;
@@ -93,6 +94,8 @@ receiver (void)
 static int
 do_test (void)
 {
+  xsignal (SIGINT, SIG_DFL);
+
   char tmp[] = "/tmp/tst-signal1-XXXXXX";
 
   int fd = mkstemp (tmp);