Exercise timer samples as part of integration tests

We need to make sure integration tests are running the timer sample as it recently failed to compile due to a regression and wasn't caught by tests.

Start by locating the integration-test setup and the timer sample in the Azure Functions host repository. Determine how existing samples are selected, add the timer sample to that coverage, and run the integration tests to verify it compiles and executes as expected.

Written by the indexing model from the issue text.