runtests: skip ld_preload tests on macOS

The LD_PRELOAD functionality doesn't exist on macOS, so skip any tests
requiring it.

Fixes #2394
Closes #3106
Reported-by: Github user @jakirkham
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
diff --git a/tests/runtests.pl b/tests/runtests.pl
index 4345df2..5dce330 100755
--- a/tests/runtests.pl
+++ b/tests/runtests.pl
@@ -2682,7 +2682,7 @@
             $curl =~ s/^(.*)(libcurl.*)/$1/g;
 
             $libcurl = $2;
-            if($curl =~ /linux|bsd|solaris|darwin/) {
+            if($curl =~ /linux|bsd|solaris/) {
                 $has_ldpreload = 1;
             }
             if($curl =~ /win32|Windows|mingw(32|64)/) {