Address flakes from using temp directory to run tests

[srujzs]

2172ba7 added support to run tests in a temporary directory.

This results in two flaky issues:

1. On Windows, build_daemon tests may fail to delete the
   temp directory because the process may have not been torn
   down yet, so it may still be accessing the file system.
   There was an initial retry after 1 second, but that appears
   to be not enough looking at a recent test run. See https://github.com/dart-lang/webdev/actions/runs/16157459808/job/45602784802?pr=2649
   for an example. It's printed as a warning.
2. If a test times out, its tearDown may not be called. In
   this case, the ResidentWebRunner in frontend_server may not
   restore the current directory in the LocalFileSystem. This
   leads to cascading failures in subsequent tests due to no
   longer being in a path that contains 'webdev'. See
   https://github.com/dart-lang/webdev/actions/runs/15989286213/job/45099373212?pr=2641
   for an example. See tearDown is not called if a test times out test#897
   as well for tracking work to call tearDown on timeouts.

To address the above issues:

1. Increase the delay between the two tries and assert this
   only occurs on Windows.
2. Cache the current directory so that it can be used in
   utilities.dart with the same (correct) value every time.