Allow control of test agent version

[bric3]

By default, the version remains test however when the environment variable TEST_AGENT_VERSION is passed the agent will use this value.

$ TEST_AGENT_VERSION="1.2.3" python3.12 -m ddapm_test_agent.agent

curl http://127.0.0.1:8126/info

with docker

$ docker build -t dd-apm-test-agent:local .
$ docker run --rm -d --name test-agent -p 8126\
    -e TEST_AGENT_VERSION="1.2.3" \
    dd-apm-test-agent:local

$ curl -s http://localhost:8126/info

[cbeauchesne]

Legit !

I'd also like to have @Kyle-Verhoog opinion of the env var name, as it'll spread in system-tests.

[Kyle-Verhoog]

lgtm! might be worth having a simple regression test so that we don't accidentally remove/change this feature but i'm good either way

[cbeauchesne]

Well, as system-tests will use it, we'll have a (slightly painful) way of being alerted 😄

[bric3]

@Kyle-Verhoog I'll merge it so progress can be made, but will come back with a test, probably next week.