Reduce Test Suite test concurrency to reduce test flakes (#3979)

# Description of Changes

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here. -->

This PR started as an experiment to see if restricting the amount of
test job threads would reduce the flakiness in the `Test Suite` CI job.
I have tested several values here (`1`, `2` and `10`) and I believe 2 is
very stable. I'm also very confused with this change because it seems
like somehow the `Test Suite` CI job is now faster than it was before:

Before:

<img width="502" height="57" alt="image"
src="https://github.com/user-attachments/assets/d944041c-8ccc-4382-a8a7-06e7ff5cba12"
/>

After:

<img width="481" height="46" alt="image"
src="https://github.com/user-attachments/assets/49b89143-b201-4a1a-820c-13e5b3feda76"
/>

It's possible that having less test threads means less cpu contention on
the github runners, but I'm not completely sure. It could also somehow
be less lock contention in SpacetimeDB. I have looked at the output of
the `Test Suite` job and it does appear that all tests are running.

# API and ABI breaking changes

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corresponding GitHub label. -->

None

# Expected complexity level and risk

0 - this only updates our workflow and has no impact on the code.

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# Testing

<!-- Describe any testing you've done, and any testing you'd like your
reviewers to do,
so that you're confident that all the changes work as expected! -->

- [x] CI is passing and `Test Suite` is less flakey

---------

Signed-off-by: John Detter <4099508+jdetter@users.noreply.github.com>

Author: jdetter

tools/ci/src/main.rs

@@ -237,7 +237,8 @@ fn main() -> Result<()> {
         Some(CiCmd::Test) => {
             // TODO: This doesn't work on at least user Linux machines, because something here apparently uses `sudo`?
 
-            cmd!("cargo", "test", "--all", "--", "--skip", "unreal").run()?;
+            // cmd!("cargo", "test", "--all", "--", "--skip", "unreal").run()?;
+            cmd!("cargo", "test", "--all", "--", "--test-threads=2", "--skip", "unreal").run()?;
             // TODO: This should check for a diff at the start. If there is one, we should alert the user
             // that we're disabling diff checks because they have a dirty git repo, and to re-run in a clean one
             // if they want those checks.