chore: parallelize unit tests across Python versions

PiperOrigin-RevId: 566350463

Author: sararob

.kokoro/presubmit/presubmit.cfg

@@ -1,10 +1,11 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
 # Run all sessions except system tests and docs builds
-# TODO(sararob): update this to unit-3.8 and change in noxfile.py after cl/565663310 is submitted
+# This only runs unit tests for Python 3.8 since unit tests are required for `cover` to run
+# Other Python version unit tests are run separately
 env_vars: {
     key: "NOX_SESSION"
-    value: "unit lint lint_setup_py blacken cover"
+    value: "unit-3.8 lint lint_setup_py blacken cover"
 }
 
 # Run unit tests in parallel, splitting up by file