[Spark] Minor test fix for partition-like data skipping (#4088)

#### Which Delta project/connector is this regarding?
- [x] Spark
- [ ] Standalone
- [ ] Flink
- [ ] Kernel
- [ ] Other (fill in here)

## Description
Currently, we look at a query's optimized plan's expressions to get
query's predicates (which we use to test data skipping). This isn't
correct - it includes the projected columns in addition to the actual
filters, which means that we might have incorrect validation (it might
be too selective). Replace this by replacing with the split conjunctive
predicates from the Filter of the query plan.

## How was this patch tested?
Test-only change.

## Does this PR introduce _any_ user-facing changes?
No

spark/src/test/scala/org/apache/spark/sql/delta/stats/PartitionLikeDataSkippingSuite.scala

@@ -27,6 +27,7 @@ import org.scalatest.BeforeAndAfter
 import org.apache.spark.SparkConf
 import org.apache.spark.sql.QueryTest
 import org.apache.spark.sql.catalyst.TableIdentifier
+import org.apache.spark.sql.catalyst.plans.logical.Filter
 import org.apache.spark.sql.functions.{array, col, concat, lit, struct}
 import org.apache.spark.sql.test.SharedSparkSession
 
@@ -72,8 +73,9 @@ trait PartitionLikeDataSkippingSuiteBase
       val res = sql(query).collect()
       assert(res.sameElements(baseResult))
 
-      val predicates =
-        sql(query).queryExecution.optimizedPlan.expressions.flatMap(splitConjunctivePredicates)
+      val predicates = sql(query).queryExecution.optimizedPlan.collect {
+        case Filter(condition, _) => condition
+      }.flatMap(splitConjunctivePredicates)
       val scanResult = DeltaLog.forTable(spark, TableIdentifier(tableName))
         .update().filesForScan(predicates)
       assert(scanResult.files.length == expectedNumFiles)