Permit concurrent unit tests (#6938)

Signed-off-by: Nick Cameron <nrc@ncameron.org>

rust/.config/nextest.toml

@@ -1,12 +1,12 @@
 [test-groups]
 # If a test uses the engine, we want to limit the number that can run in parallel.
 # This way we don't start and stop too many engine instances, putting pressure on our cloud.
-uses-engine = { max-threads = 4 }
+uses-engine = { max-threads = 32 }
 # If a test must run after the engine tests, we want to make sure the engine tests are done first.
-after-engine = { max-threads = 12 }
+after-engine = { max-threads = 32 }
 
 [profile.default]
-slow-timeout = { period = "180s", terminate-after = 1 }
+slow-timeout = { period = "280s", terminate-after = 1 }
 
 [profile.ci]
 slow-timeout = { period = "280s", terminate-after = 5 }