Use explicit pg major versions in alternative outputs

When we get a different output for a test with different pg major, we use an alternative output to make the tests pass such as `test_output_0.out`. We currently have lots of alternative outputs and with the `0` suffix it is hard to know which pg major version's output the file belongs to.

For example while dropping pg-11, we didn't know which alternative outputs were for only pg-11.

I think it would make sense to use the pg major as a suffix instead of `0`, `1` etc. So something like `test_output-12.out` is more understandable than `test_output-0.out`. The other file will be `test_output-13.out` rather than `test_output.out`. In case there is no alternative output file, we can just keep it as `test_output.out`.

Since we only have pg12 now, it should be easy to do this conversion on existing files with a script