runtests.pl: check for tstunnel command on Windows

The Windows console version of stunnel is called "tstunnel", while
running "stunnel" on Windows spawns a new console window which
cannot be handled by the testsuite.

tests/runtests.pl

@@ -191,7 +191,7 @@ my $pwd = getcwd();          # current working directory
 my $start;
 my $ftpchecktime=1; # time it took to verify our test FTP server
 
-my $stunnel = checkcmd("stunnel4") || checkcmd("stunnel");
+my $stunnel = checkcmd("stunnel4") || checkcmd("tstunnel") || checkcmd("stunnel");
 my $valgrind = checktestcmd("valgrind");
 my $valgrind_logfile="--logfile";
 my $valgrind_tool;