Run model tests and proptests at lower concurrency in CI (#27364)

This seems to offer a modest speedup, between 20-40s depending on the machine type.
The model tests hammer local postgres too quickly and use up all available disk I/O, and proptests peg the CPU. By limiting the number that can be running from each group, we are able to make forward progress a bit faster.

GitOrigin-RevId: d6c7608166427780b3dfe7ef6194adf283a324de

Author: gautamg795

.config/nextest.toml

@@ -5,3 +5,14 @@ path = "junit.xml"
 fail-fast = false
 failure-output = "immediate-final"
 
+[[profile.ci.overrides]]
+filter = 'test(/model::tests::model_tests::/)'
+test-group = 'model-tests'
+
+[[profile.ci.overrides]]
+filter = 'test(/roundtrips/) or test(/proptest/)'
+test-group = 'proptests'
+
+[test-groups]
+model-tests = { max-threads = 4 }
+proptests = { max-threads = 4 }