test(pack): refactor 'pack_spec.lua' with better code

Problem: 'pack_spec.lua' test file's code can be improved.

Solution: Refactor some aspects. In particular:
  - Rewrite `find_in_log` for finding event data in event log into a
    generator function that from log list returns a finder function.
    This allows it to take less arguments and be more concise.

  - Consistently use `local function f()` instead of
    `local f = function()`.

  - Prefer to use `fn.readblob()` instead of `fn.readfile()` to assert
    text from a file.

  - Use `([[...]]):format()` approach to testing file content (instead
    of array of strings). Should improve readability.

  - Universally prefer using "assert" to mean "check if certain
    expectation about the process holds up" (instead of occasional
    "validate").

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