Optionally use an on-disk sqlite db in tests (#11702)

* Optionally use an on-disk sqlite db in tests

When debugging a test it is sometimes useful to inspect the state of the
DB. This is not easy when the db is in-memory: one cannot attach the
sqlite CLI to another process's DB.

With this change, if SYNAPSE_TEST_PERSIST_SQLITE_DB is set, we use
 `_trial_temp/test.db` as our sqlite database. One can then use 
`sqlite3 _trial_temp/test.db` and query to your heart's content.

The DB is destroyed and recreated between different test cases.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/11702.misc b/changelog.d/11702.misc
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+Add the option to write sqlite test dbs to disk when running tests.
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