chore: improve integration tests

[PrettyWood]

Feedbacks on integration tests

When I first started to work on integration tests, I didn't know there was the great
prqlc/prql-compiler/tests/integration/dbs/README.md file that explains clearly how to run those tests.
I first had an issue by running

cargo nextest run --all-targets -F "test-dbs,test-dbs-external

and got bitten by the "it doesn't handle multiple processes"
Fortunately I saw the message

# test-dbs runs database setup when the connection is established,
# and because nextest runs test in separate processes, this happens on every test.
# To prevent mutliple setups running at once, we set max-threads to 1.

in nextest.toml and managed to run them properly

I also had issues with just starting the mssql container but this is due to my M1 and I found this issue that was already
linked in the docker-compose (microsoft/mssql-docker#668 (comment))
Thank you for that!

Once MSSQL was up and running, I thought everything would work. I was wrong 😬

The first time I run the test I got a timeout on the MSSQL server

test queries::results::aggregation has been running for over 60 seconds
.test queries::results::aggregation ... FAILED

failures:

---- queries::results::aggregation stdout ----
thread 'queries::results::aggregation' panicked at 'called `Result::unwrap()` on an `Err` value: Tls("connection closed via error")', prqlc/prql-compiler/tests/integration/dbs/protocol/mssql.rs:32:18

I also got once the error

thread 'queries::results::aggregation' panicked at 'Failed to insert row: no such table: albums.my

Caused by:
    Error code 1: SQL error or missing database

I don't remember if I stopped a test during execution or whatever but I had to dig a bit in the code
that it's just a side effect of the MySQL test that copies the csv files into xyz.my.csv.
And since they are git ignored, you don't see anything in a git status.
Anyway easy fix:

find . -type f -name '*.my.csv' -delete

So how do we fix the MSSQL timeout error?

It appears there is an issue with the TLS handshake on M1 (see prisma/tiberius#65)
I propose the switch to rustls since it doesn't require headers or runtime libraries.
(it conflicts with async_native_tls and is in the default features henece the need to deactive
those: see prisma/tiberius#317)

And when I went with this approach the handshake was done! Great so it works finally!!!
OR NOT

error[E0277]: the trait bound `BigDecimal: tiberius::FromSql<'_>` is not satisfied
   --> prqlc/prql-compiler/tests/integration/dbs/protocol/mssql.rs:64:48
    |
64  | ...                   .get::<BigDecimal, usize>(i)
    |                                          ^^^^^ the trait `tiberius::FromSql<'_>` is not implemented for `BigDecimal`
    |
    = help: the following other types implement trait `tiberius::FromSql<'a>`:
              bool
              i16
              i32
              i64
              u8
              f32
              f64
              Uuid
            and 5 others
note: required by a bound in `tiberius::Row::get`
   --> /Users/ericjolibois/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tiberius-0.12.2/src/row.rs:375:12
    |
373 |     pub fn get<'a, R, I>(&'a self, idx: I) -> Option<R>
    |            --- required by a bound in this associated function
374 |     where
375 |         R: FromSql<'a>,
    |            ^^^^^^^^^^^ required by this bound in `Row::get`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `prql-compiler` (test "integration") due to previous error

Ahhhh right! By setting default-features = false, I forgot to add tds73 which is also enabled by default
Annnnnnd TADA!

Improvements

When you run a test and see

test queries::compile::math ... ok

you don't know which dialects have been tested
And with a little dbg!() macro you see that MSSQL is not tested by default!

[prqlc/prql-compiler/tests/integration/queries.rs:164] test_name = "math"
[prqlc/prql-compiler/tests/integration/queries.rs:164] dialect = DuckDb
[prqlc/prql-compiler/tests/integration/queries.rs:164] test_name = "math"
[prqlc/prql-compiler/tests/integration/queries.rs:164] dialect = Postgres
[prqlc/prql-compiler/tests/integration/queries.rs:164] test_name = "math"
[prqlc/prql-compiler/tests/integration/queries.rs:164] dialect = MySql
[prqlc/prql-compiler/tests/integration/queries.rs:164] test_name = "math"
[prqlc/prql-compiler/tests/integration/queries.rs:164] dialect = ClickHouse
[prqlc/prql-compiler/tests/integration/queries.rs:164] test_name = "math"
[prqlc/prql-compiler/tests/integration/queries.rs:164] dialect = GlareDb

So you NEED to add # mssql:test on top of the file to run it. That's weird compared to the others...
But then I looked at the code and saw that level is "UNSUPPORTED". So makes sense but again you need to know to know ;)

And the second point is just to improve/update a bit the documentation

---

So I would go with those changes in this PR if you're ok with it

• make integration tests pass on M1 (for that I would like to know if it doesn't break anything for tests on Linux)
• improve documentation

[PrettyWood]

@max-sixty Here you go 😄

[max-sixty]

Thanks a lot for the report, and for the the PR!