Some notable differences:

* For the last "appendonly_verify_block_checksums_co" test, don't use a
  compressed table. That seems fragile.

* In the "appendonly_verify_block_checksums_co" test, be more explicit in
  what is replaced. It used to scan backwards from end of file for the byte
  'a', but we didn't explicitly include that byte in the test data. What
  actually gets replaced depends heavily on how integers are encoded. (And
  the table was compressed too, which made it even more indeterministic.)
  In the rewritten test, we replace the string 'xyz', and we use a text
  field that contains that string as the table data.

* Don't restore the original table file after corrupting. That seemed
  uninteresting to test. Presumably the table was OK before we corrupted
  it, so surely it's still OK after restoring it back. In theory, there
  could be a problem if the file's corrupt contents were cached somewhere,
  but we don't cache AO tables, and I'm not sure what we'd try to prove by
  testing that, anway, because swapping the file while the system is active
  is surely not supported.

* The old script checked that the output when a corrupt table was SELECTed
  from contained the string "ERROR:  Header checksum does not match".
  However, half of the tests actually printed a different error, "Block
  checksum does not match". It turns out that the way the old select_table
  function asserted the result of the grep command was wrong. It should've
  done "assert(int(result) > 0), ..." rather than just "assert(result > 0),
  ...". As written, it always passed, even if there was no ERROR in the
  output. The rewritten test does not have that bug.