hscontrol/policy/v2: rename tailscale_compat_test.go to tailscale_acl_compat_test.go

Rename the ACL compatibility test file to include 'acl' in the name,
making room for the upcoming grants compatibility test file.

Also fix a godoclint issue by adding a blank line between the file
header comment and the package declaration.

Updates #2180

hscontrol/policy/v2/tailscale_acl_compat_test.go

@@ -13,6 +13,7 @@
 // and improve our policy implementation.
 // This file is NOT intended for developer/humans to change and should be
 // consider a "black box" test suite.
+
 package v2
 
 import (