Reduce nextest threads to 8 (#4846)

## Issue Addressed

Fix OOMs caused by too many concurrent tests. The runner machine is currently liable to run `32 * 5 = 160` tests in parallel. If each test uses say 300MB max, this is 48GB of RAM!

## Proposed Changes

Reduce the number of threads per runner job to 8. This should cap the memory at 4x lower than the current limit, i.e. around 12GB. If we continue to run out of RAM, we should consider more sophisticated limits.

.config/nextest.toml

@@ -20,7 +20,7 @@ retries = 0
 
 # The number of threads to run tests with. Supported values are either an integer or
 # the string "num-cpus". Can be overridden through the `--test-threads` option.
-test-threads = "num-cpus"
+test-threads = 8
 
 # The number of threads required for each test. This is generally used in overrides to
 # mark certain tests as heavier than others. However, it can also be set as a global parameter.