[test] Recover from a leftover build process on test retry
When a `next start` test exceeds the per-test timeout while `next build`
is still running, the build process is left running and tracked by the
next instance, because the build phase of `NextStartInstance.start()`
clears `childProcess` only when the process exits. `jest.retryTimes`
then re-runs the test body in the same process, and `start()`
immediately throws `next already started`, which masks the original
timeout.
`NextStartInstance` now tracks whether a build or a server currently
owns the child process. When `start()` finds a leftover `next build`
process from an interrupted attempt, it stops that process and continues
instead of throwing, so the retry can start from a clean state. The
`next already started` guard is preserved for the case where a server is
genuinely already running.
