tests: meson: Add note about guarding the transport tests

The transport tests rely on a 'test' transport implementation that will
never be marked as stable.

Change-Id: Ib7766c25a435d1bd27f13d45a50018806b225cae
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/tests/meson.build b/tests/meson.build
index 4ba170f..04dd4dc 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -27,6 +27,8 @@
   'responder',
 ]
 
+# The 'test' transport APIs will never be marked stable as they are just for
+# testing.
 if get_option('abi').contains('testing')
   tests += [
     'transport',