patch 9.0.1951: Vim9: hard to debug vim9_class errors from CI

Problem:  Vim9: hard to debug vim9_class errors from CI
Solution: Include the line number in assert_xxx() calls.  Include the
          entire error message in the tests.  Fix the indentation in the
          test file.  Add tags for new error codes.

closes: #13206

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/vim9class.c b/src/vim9class.c
index 4c70f8d..29ed6d1 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -1577,7 +1577,7 @@
 	    }
 	    if (!is_class)
 	    {
-		emsg(_(e_public_member_not_supported_in_interface));
+		emsg(_(e_public_variable_not_supported_in_interface));
 		break;
 	    }
 	    has_public = TRUE;