Add back UPB test coverage that was undone from test migration.

I've been gradually moving the location of our UPB tests to make them "more
default" (see an example here:
a02ec0f)

It turns out that reduced some of our open-source coverage around UPB python
unit tests. In this commit, I temporarily hard-code the tests I've migrated,
and eventually I'll change it into a wildcard expansion to be more robust. We
can't do wildcards right now because not all tests in the
google.protobuf.internal namespace support UPB by default yet.

#test-continuous

PiperOrigin-RevId: 707564382

.github/workflows/test_upb.yml

@@ -264,13 +264,24 @@ jobs:
       - name: Install Protobuf Test Wheel
         if: ${{ !matrix.continuous-only || inputs.continuous-run }}
         run: pip install -vvv --no-index --find-links wheels protobuftests
-      - name: Run the unit tests
+      - name: Run legacy unit tests
         if: ${{ !matrix.continuous-only || inputs.continuous-run }}
+        # Legacy upb tests use a "test wrapper" and get installed under pb_unit_tests.
         run: |
-          TESTS=$(pip show -f protobuftests | grep pb_unit_tests.*py$ | sed 's,/,.,g' | sed 's,\\,.,g' | sed -E 's,.py$,,g')
-          for test in $TESTS; do
+          PB_UNIT_TESTS=$(pip show -f protobuftests | grep pb_unit_tests.*py$ | sed 's,/,.,g' | sed 's,\\,.,g' | sed -E 's,.py$,,g')
+          for test in $PB_UNIT_TESTS; do
             python -m unittest -v $test
           done
+      - name: Run unit tests
+        if: ${{ !matrix.continuous-only || inputs.continuous-run }}
+        # Newer upb tests are in the standard google.protobuf.internal path.
+        # We will eventually make this into a wildcard rule once all tests
+        # have been migrated to be compatible with upb.
+        run: |
+          TESTS=(message_test message_factory_test descriptor_test proto_builder_test)
+          for test in ${TESTS[@]}; do
+            python -m unittest -v google.protobuf.internal.${test}
+          done
 
   test_pure_python_wheels:
     name: Test Pure Python Wheels