revert

Author: parthea

.kokoro/presubmit/presubmit.cfg

@@ -1,7 +1,7 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
-# Disable system tests.
+# Only run a subset of all nox sessions
 env_vars: {
-    key: "RUN_SYSTEM_TESTS"
-    value: "false"
-}
+    key: "NOX_SESSION"
+    value: "unit-3.8 unit-3.12 cover docs docfx"
+}