bisync: speed up integration tests on non-local remotes - #10016

What does this change do?

bisync: speed up integration tests on non-local remotes

Before this change, the bisync integration tests checked whether each remote could set modtimes on every bisync step (checkPreReqs, called from runBisync), not once per remote. On a remote that isn't IsLocal, each check sleeps for 1s twice (the Google Cloud Storage per-object rate limit workaround from be73a10), for each side, on top of the check's own API calls. That added 2-4s to each of the ~110 bisync steps in each test.

After this change, the result of the check is remembered for each remote, so it runs, and sleeps, once per remote per test run. The sleeps themselves are unchanged.

bisync: fix Dropbox rate limit notices failing the integration tests

The tests already drop Dropbox's "Too many requests or write operations" notice from the logs, but only in the format it had when the tests were written. Since 110bf46 (v1.65), the notice begins with Error , so the filter stopped matching. Without the sleeps above, the tests run faster and are more likely to be rate limited. This change widens the pattern to match either format.

Benchmarks

Last night's integration test logs, next to runs with this PR:

The "with this PR" runs were on my machine, so only the SFTP row is a like-for-like comparison: master took 1007s there, against 66s with this PR. For Drive and Dropbox, some of the difference is network and account, not this change.

The Dropbox run had four failures, none caused by this change:

• One was the rate limit notice that the second commit now filters.
• The other three were Dropbox server errors (HTTP 500 and "unexpected error occurred"), all within 13 minutes of each other.

Those four cases passed on a rerun, both with this PR and on master.

Testing

• go test ./cmd/bisync -race (local): passes
• -remote :memory:: 6.8s
• -remote TestCrypt:: 24.7s
• TestSFTPRclone:, TestDrive:, TestDropbox:: as above

Linked issue

No issue -- I found this while measuring bisync test coverage against more remotes.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester - see CONTRIBUTING.md.
• This Pull Request is ready for review.