tests: fix gnutls-serv check

- If gnutls-serv doesn't exist then don't try to execute it.

Follow-up to 2fdc1d81.

Closes https://github.com/curl/curl/pull/10688

tests/sshhelp.pm

@@ -409,15 +409,17 @@ sub find_sshkeygen {
 #
 sub find_httptlssrv {
     my $p = find_exe_file_hpath($httptlssrvexe);
-    my @o = `$p -l`;
-    my $found;
-    for(@o) {
-        if(/Key exchange: SRP/) {
-            $found = 1;
-            last;
+    if($p) {
+        my @o = `"$p" -l`;
+        my $found;
+        for(@o) {
+            if(/Key exchange: SRP/) {
+                $found = 1;
+                last;
+            }
         }
+        return $p if($found);
     }
-    return $p if($found);
 }