test.pl: Don't use watchdog thread unless safe signals

In the thread documentation, I noticed that unsafe signals preclude
killing threads with a signal.  But this is the normal method for
clearing a watchdog timer under threads.  I doubt that people are
compiling with PERL_OLD_SIGNALS these days, so I didn't check for that,
but its easy enough to check for the environment variable that does the
same thing at runtime.

Author: khwilliamson

t/test.pl

@@ -1888,9 +1888,12 @@ ($;$)
     # shut up use only once warning
     my $threads_on = $threads::threads && $threads::threads;
 
-    # Use a watchdog process unless 'threads' is loaded
-    if (!$threads_on || $method eq "process") {
-
+    # Use a watchdog process unless 'threads' is loaded and is killable by a
+    # signal
+    if (   ! $threads_on
+        || (defined $ENV{PERL_SIGNALS} && $ENV{PERL_SIGNALS} eq "unsafe")
+        || $method eq "process")
+    {
         # On Windows and VMS, try launching a watchdog process
         #   using system(1, ...) (see perlport.pod).  system() returns
         #   immediately on these platforms with effectively a pid of the new