added more tests and cleanup

closes #20

added more tests and cleanup

closes #20

…ateExpression

### What changes were proposed in this pull request?

This pr intends to add filter information in the explain output of an aggregate (This is a follow-up of apache#26656).

Without this pr:
```
scala> sql("select k, SUM(v) filter (where v > 3) from t group by k").explain(true)
== Parsed Logical Plan ==
'Aggregate ['k], ['k, unresolvedalias('SUM('v, ('v > 3)), None)]
+- 'UnresolvedRelation [t]

== Analyzed Logical Plan ==
k: int, sum(v): bigint
Aggregate [k#0], [k#0, sum(cast(v#1 as bigint)) AS sum(v)#3L]
+- SubqueryAlias `default`.`t`
   +- Relation[k#0,v#1] parquet

== Optimized Logical Plan ==
Aggregate [k#0], [k#0, sum(cast(v#1 as bigint)) AS sum(v)#3L]
+- Relation[k#0,v#1] parquet

== Physical Plan ==
HashAggregate(keys=[k#0], functions=[sum(cast(v#1 as bigint))], output=[k#0, sum(v)#3L])
+- Exchange hashpartitioning(k#0, 200), true, [id=#20]
   +- HashAggregate(keys=[k#0], functions=[partial_sum(cast(v#1 as bigint))], output=[k#0, sum#7L])
      +- *(1) ColumnarToRow
         +- FileScan parquet default.t[k#0,v#1] Batched: true, DataFilters: [], Format: Parquet, Location: InMemoryFileIndex[file:/Users/maropu/Repositories/spark/spark-master/spark-warehouse/t], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<k:int,v:int>

scala> sql("select k, SUM(v) filter (where v > 3) from t group by k").show()
+---+------+
|  k|sum(v)|
+---+------+
+---+------+
```

With this pr:
```
scala> sql("select k, SUM(v) filter (where v > 3) from t group by k").explain(true)
== Parsed Logical Plan ==
'Aggregate ['k], ['k, unresolvedalias('SUM('v, ('v > 3)), None)]
+- 'UnresolvedRelation [t]

== Analyzed Logical Plan ==
k: int, sum(v) FILTER (v > 3): bigint
Aggregate [k#0], [k#0, sum(cast(v#1 as bigint)) filter (v#1 > 3) AS sum(v) FILTER (v > 3)#5L]
+- SubqueryAlias `default`.`t`
   +- Relation[k#0,v#1] parquet

== Optimized Logical Plan ==
Aggregate [k#0], [k#0, sum(cast(v#1 as bigint)) filter (v#1 > 3) AS sum(v) FILTER (v > 3)#5L]
+- Relation[k#0,v#1] parquet

== Physical Plan ==
HashAggregate(keys=[k#0], functions=[sum(cast(v#1 as bigint))], output=[k#0, sum(v) FILTER (v > 3)#5L])
+- Exchange hashpartitioning(k#0, 200), true, [id=#20]
   +- HashAggregate(keys=[k#0], functions=[partial_sum(cast(v#1 as bigint)) filter (v#1 > 3)], output=[k#0, sum#9L])
      +- *(1) ColumnarToRow
         +- FileScan parquet default.t[k#0,v#1] Batched: true, DataFilters: [], Format: Parquet, Location: InMemoryFileIndex[file:/Users/maropu/Repositories/spark/spark-master/spark-warehouse/t], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<k:int,v:int>

scala> sql("select k, SUM(v) filter (where v > 3) from t group by k").show()
+---+---------------------+
|  k|sum(v) FILTER (v > 3)|
+---+---------------------+
+---+---------------------+
```

### Why are the changes needed?

For better usability.

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

Manually.

Closes apache#27198 from maropu/SPARK-27986-FOLLOWUP.

Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>