Based on some helpful feedback, I'm recasting my question. Sorry for the do-over.
Problem: The proliferation of duplicate src/Makefile for policy_* tests, and their resultant python protobuf binding files, is a) increasing the size of the autotest directory download to DUTs, and b) lengthening the time it takes to emerge autotests. A single src directory grows to about 573K after Makefile is run. With 22 tests (currently), this ends up consuming 12.54 MB. As many more tests are coming, this problem will only get worse. Is this a valid concern? If not, let me know, and I'll stop trying to fix it.
If this is valid a concern, then my idealized solution is to make all policy_* tests, and the policy_testserver they invoke, share a common src directory that contains the Makefile and compiled protobuf binding files. This way, only one instance of the src directory needs to exist, get compiled, and be copied down to the DUT, when any policy_* test is run. Is this a realistic solution? If not, please let me know what would be.
Solution: Assuming my proposed solution is feasible, there are two bits of news I think are good:
1) the policy_testserver.py is already in the test image, at /usr/local/telemetry/src/chrome/browser/policy/test, and
2) the policy *.proto files are already in the test image, at /usr/local/share/protofiles and /user/share/protofiles.
This leaves at least two programming problems I need to solve. Both are beyond my current knowledge. They are:
1) How do I get build_packages (and emerge) to execute the Makefile stored in the shared source location (e.g., .../src/third_party/autotest/autotest/files/client/cros/enterprise/src/Makefile), so that it creates therein the protobuf binding files (e.g., cloud_policy_pb2.py and .pyc, chrome_device_policy_pb2.py and .pyc, etc...)
2) How do I make policy_testserver.py import the protobuf binding files from *that* location, rather than from the self.srcdir directory of the running test.