screen.lua: expect_unchanged(), get_snapshot()

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".

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