pull a list of pushdowns to witness from each flux e2e test and require post-run

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 .require_pushdown (or similar) member. Call this function in influxdb e2e test harness and verify that the pushdown occurred. The mechanism to ensure it occurred could either be using the meta channel (Adding Metadata to source nodes), or otherwise logging which rewrite rules occured.

DOD
- Can specify which pushdowns should have occurred from individual test cases and the test harness enforces this.

Start with flux's stdliib/testing.go and the InfluxDB e2e test harness. Trace how the AST exposes each test's _columns = () => statement and compare the metadata-channel and rewrite-rule logging options for confirming pushdowns. Done means individual tests can declare expected pushdowns and the harness enforces that they occurred.

Written by the indexing model from the issue text.