runtests: assume Time::HiRes, drop Perl Win32 dependency

`Time::HiRes` was already used unconditionally before this patch in
`servers.pm`. This package, and functions used by runtests (`sleep` and
`gettimeofday`) are supported by the minimum Perl version required for
curl:

https://perldoc.perl.org/5.8.0/Time::HiRes

- Drop the `portable_sleep()` wrapper in favor of `Time::HiRes::sleep()`.
- Use `Time::HiRes` unconditionally in `serverhelp.pm`.
- Stop using the `Win32` package where available. It was included
  to provide a Windows fallback for `Time::HiRes::sleep()`. It was never
  actually called, but the dependency may have loaded `Win32.dll`, which
  often appears in failed fork operations in GHA logs.
  Ref: a6fed41f6f #5054 #5034
  Ref: https://github.com/curl/curl/discussions/14854

Closes #18287

tests/ftpserver.pl

@@ -51,6 +51,7 @@ BEGIN {
 use IPC::Open2;
 use Digest::MD5;
 use File::Basename;
+use Time::HiRes;
 
 use directories;
 
@@ -484,7 +485,7 @@ sub sendcontrol {
 
         for(@a) {
             sockfilt $_;
-            portable_sleep($ctrldelay);
+            Time::HiRes::sleep($ctrldelay);
         }
     }
     my $log;
@@ -521,7 +522,7 @@ sub senddata {
             # pause between each byte
             for (split(//,$l)) {
                 sockfiltsecondary $_;
-                portable_sleep($datadelay);
+                Time::HiRes::sleep($datadelay);
             }
         }
     }
@@ -3292,7 +3293,7 @@ while(1) {
             logmsg("Sleep for $delay seconds\n");
             my $twentieths = $delay * 20;
             while($twentieths--) {
-                portable_sleep(0.05) unless($got_exit_signal);
+                Time::HiRes::sleep(0.05) unless($got_exit_signal);
             }
         }