sql/logictest: implement "subtest end"

The [datadriven](https://github.com/cockroachdb/datadriven/blob/master/datadriven.go#L233-L235) package supports ending subtests with the "subtest end" command. We use this prolifically in logictests, even though there is no explicit support for this command - it simply starts a new subtest named "end". We should correctly support "subtest end" with the behavior:

1. All subtests must be explicitly "ended".
2. Subtests cannot have the name "end".

Jira issue: CRDB-42059