Tests time out on CircleCI

[hacklschorsch]

In ​https://github.com/tahoe-lafs/tahoe-lafs/pull/1381#issuecomment-2476885548 meejah writes:

The unclean-reactor errors may be simply a downstream symptom of the real errors that also happen in that run (e.g. several tests time out).

My own tests suggest that indeed, raising the SystemTests? timeout make ​a couple of flaky tests much more stable:

Failure count	Test name
1	allmydata.test.test_system.HTTPSystemTest.test_mutable_mdmf
3	allmydata.test.test_system.HTTPSystemTest.test_mutable_sdmf
30	allmydata.test.test_system.HTTPSystemTest.test_upload_and_download_convergent
11	allmydata.test.test_system.HTTPSystemTest.test_upload_and_download_random_key

This ticket is similar but not equal to ticket:4085, ticket:4022, ticket:2994 .