if there are nested tests and nested servers, we need to restore the
previous value of cur_test when a test exist.
example:
```
test{test 1} {
start_server {
test{test 1.1 - master only} {
}
start_server {
test{test 1.2 - with replication} {
}
}
}
}
```
when `test 1.1 - master only exists`, we're still inside `test 1`
(cherry picked from commit 610b4ff16a62062338588c4508a73784fb962c0b)
826b49bcb4