fix(flink): fix compilation of over aggregation query in flink backend

[chloeh13q]

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Description of changes

This PR fixes the compilation of over aggregation query in Flink backend.

I identified the following issues:

1. The interval time value is not compiled with the appropriate precision. ibis.interval(minutes=500) compiles to `INTERVAL '500' MINUTE, which throws

org.apache.calcite.sql.validate.SqlValidatorException: Interval field value 500 exceeds precision of MINUTE(2) field

2. range=(-ibis.interval(minutes=500), 0) compiles to [...] AND CAST(0 AS INTERVAL MINUTE) following and Flink doesn't support rows/range following in over aggregation. This throws

org.apache.flink.table.api.TableException: OVER RANGE FOLLOWING windows are not supported yet.

I also removed the unused test snapshot directory that didn't get cleaned up.

Issues closed