Run datastore tests against multiple schemas (#1277)

We want to prove in tests that some Janus version can run safely on
multiple schema versions to make database schema migrations safer. In
this commit:

 - `aggregator_core::test_util::EphemeralDatastore` can now be
   constructed with a `max_schema_version` argument to control which
   migration scripts are applied during tests
 - `datastore::SUPPORTED_SCHEMA_VERSIONS` is now emitted by the
   `supported_schema_versions` macro, which also constructs a template
   for stamping out tests.
 - We adopt [`rstest`][1] to inject an `EphemeralDatastore` instance
   into tests in `aggregator_core::datastore::Datatore::tests`. Using
   [`rstest_reuse`][2], we can automatically stamp out versions of
   existing tests that run using multiple schema versions.

We only use this dependency injection technique in the `datastore`
module, because that should be the only module that's tightly coupled to
the database schema. We could run all tests that use a datastore this
way, at the cost of increasing overall test runtime.

[1]: https://docs.rs/rstest
[2]: https://docs.rs/rstest_reuse

aggregator_core/Cargo.toml

@@ -62,9 +62,12 @@ uuid = { version = "1.3.1", features = ["v4"] }
 
 [dev-dependencies]
 assert_matches = "1"
+async-std = { version = "1.12.0",  features = ["attributes"] }
 hyper = "0.14.26"
 janus_aggregator_core = { path = ".", features = ["test-util"] }
 janus_core = { workspace = true, features = ["test-util"] }
+rstest = "0.17.0"
+rstest_reuse = "0.5.0"
 serde_test = "1.0.160"
 tempfile = "3.5.0"
 tokio = { version = "1", features = ["test-util"] }  # ensure this remains compatible with the non-dev dependency