Waits for a current transaction to finish in TEST_CompactRange when the
`background_work_finished_signal_` signal may be emitted due to an error. This is a prospective fix for flaky crashes observed in b/351680196#comment5.

PiperOrigin-RevId: 663457574

There are a couple differences between ChromiumEnv and
PosixEnv/WindowsEnv which cause test failures that are fixed (or at
least patched over) in this change:

* NewSequentialFile() and NewRandomAccessFile() return Status::IOError
  rather than Status::NotFound when a file is not found, due to
  https://crbug.com/760362. This means a few tests need to expect a
  different error result.
* GetChildren() never returns the '.' or '..' entries.
* As allowed by the documentation for Env::Schedule(), ChromiumEnv may
  execute functions on multiple threads and guarantees no sequencing.
  EnvTest.RunMany assumed that functions ran in order. The test has been
  updated.