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/runner.pm

@@ -39,6 +39,7 @@ use warnings;
 use 5.006;
 
 use File::Basename;
+use Time::HiRes;
 
 BEGIN {
     use base qw(Exporter);
@@ -84,9 +85,6 @@ use Storable qw(
 use pathhelp qw(
     exe_ext
     );
-use processhelp qw(
-    portable_sleep
-    );
 use servers qw(
     checkcmd
     initserverconfig
@@ -419,7 +417,7 @@ sub waitlockunlock {
         my $lockretry = $serverlogslocktimeout * 20;
         my @locks;
         while((@locks = logslocked()) && $lockretry--) {
-            portable_sleep(0.05);
+            Time::HiRes::sleep(0.05);
         }
         if(($lockretry < 0) &&
            ($serverlogslocktimeout >= $defserverlogslocktimeout)) {
@@ -1092,7 +1090,7 @@ sub singletest_clean {
         }
     }
 
-    portable_sleep($postcommanddelay) if($postcommanddelay);
+    Time::HiRes::sleep($postcommanddelay) if($postcommanddelay);
 
     my @killtestservers = getpart("client", "killserver");
     if(@killtestservers) {