Split integration tests into separate files

[pwithams]

PR for #1744.

Total test runtime appears very slightly slower (by ~100ms for me locally), presumably a constant due to the overhead of creating separate binaries for each test file. However, I think the improved organization is probably worth it as it makes it a little clearer how the tests are distributed and to know where to put new tests.

As each test file is created as a separate binary now clippy warns when parts of testenv are not used by every test file, so I added #![allow(dead_code)] at the top of testenv/mod.rs to suppress that. Not sure if there's a better way to handle that.

I'm open to any suggestions for different file layouts too. For example, if there any gaps in current test types a file could be created to highlight that with the goal of filling it out.

[tmccombs]

We could potentially make a single create with mutiple files for the integration tests, like https://momori.dev/posts/organize-rust-integration-tests-without-dead-code-warning/#one-integration-test-crate-with-modules

I'm not sure if the is the right thing to do here or not.

[pwithams]

Yeah I like that idea. I see that cargo uses a similar approach:

rust-lang/cargo#5022
https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html

Some timings after reorganizing - seems runtime is looking good again:

Original HEAD: cargo test 3.70s user 1.46s system 539% cpu 0.956 total
Splitting tests into separate files/binaries: cargo test 3.51s user 1.40s system 448% cpu 1.096 total
Using a single crate for tests: cargo test 3.74s user 1.39s system 584% cpu 0.879 total

I renamed the original tests/tests.rs file to tests/integration/general.rs to try make it clear each file is a related grouping of tests.