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 it's easy enough to check for the environment variable that does the same thing at runtime.

Author: khwilliamson

t/test.pl

@@ -1890,9 +1890,12 @@ ($;$)
  # 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