Factor `get_snapshot()` out of `print_snapshot()`, so that callers can
get a table (for use with `expect()`) instead of the string form.
Try to use this to fix indeterminism in `searchhl_spec.lua`.
- Since the screen state is collected by `screen:expect_unchanged()`,
we don't need a deterministic initial state (which would then be
hardcoded into the test). This allows us to check "did anything
change in the last N ms?" rather than "did anything change compared
to a hardcoded screen-state?"
- This may end up fruitless, because `expect_unchanged()` depends on
timing to wait for an initial "current state".