subversion_spec: fix svnadmin calls on Linux.

Ensure that we only use `xcrun` on Catalina or newer.

Library/Homebrew/test/unpack_strategy/subversion_spec.rb

@@ -8,7 +8,9 @@ describe UnpackStrategy::Subversion, :needs_svn do
   let(:path) { working_copy }
 
   before do
-    safe_system "xcrun", "svnadmin", "create", repo
+    svnadmin = ["svnadmin"]
+    svnadmin = ["xcrun", *svnadmin] if OS.mac? && MacOS.version >= :catalina
+    safe_system(*svnadmin, "create", repo)
     safe_system "svn", "checkout", "file://#{repo}", working_copy
 
     FileUtils.touch working_copy/"test"