pull a list of feature flags to enable from each flux e2e test

Add a function to flux's `stdliib/testing.go` that walks the AST, finds the `test _columns = () =>` statements, calls the function and returns the list of strings on the `.enable_ff` (or similar) member. Call this function in influxdb e2e test harness and set the flags in the feature flagger defined there.

DOD:
- Can enable feature flags from test cases.

Start with flux's stdliib/testing.go and the InfluxDB e2e test harness mentioned in the issue; trace how test cases and the feature flagger are currently handled. Done means the harness discovers each test's enable_ff values and enables those feature flags when the tests run.

Written by the indexing model from the issue text.