[SPARK-33265][TEST] Rename classOf[Seq] to classOf[scala.collection.Seq] in PostgresIntegrationSuite for Scala 2.13

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

This PR renames some part of `Seq` in `PostgresIntegrationSuite` to `scala.collection.Seq`.
When I run `docker-integration-test`, I noticed that `PostgresIntegrationSuite` failed due to `ClassCastException`.
The reason is the same as what is resolved in SPARK-29292.

### Why are the changes needed?

To pass `docker-integration-test` for Scala 2.13.

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

No.

### How was this patch tested?

Ran `PostgresIntegrationSuite` fixed and confirmed it successfully finished.

Closes apache#30166 from sarutak/fix-toseq-postgresql.

Authored-by: Kousuke Saruta <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: sarutak

external/docker-integration-tests/src/test/scala/org/apache/spark/sql/jdbc/PostgresIntegrationSuite.scala

@@ -116,14 +116,14 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
     assert(classOf[java.lang.Boolean].isAssignableFrom(types(7)))
     assert(classOf[String].isAssignableFrom(types(8)))
     assert(classOf[String].isAssignableFrom(types(9)))
-    assert(classOf[Seq[Int]].isAssignableFrom(types(10)))
-    assert(classOf[Seq[String]].isAssignableFrom(types(11)))
-    assert(classOf[Seq[Double]].isAssignableFrom(types(12)))
-    assert(classOf[Seq[BigDecimal]].isAssignableFrom(types(13)))
+    assert(classOf[scala.collection.Seq[Int]].isAssignableFrom(types(10)))
+    assert(classOf[scala.collection.Seq[String]].isAssignableFrom(types(11)))
+    assert(classOf[scala.collection.Seq[Double]].isAssignableFrom(types(12)))
+    assert(classOf[scala.collection.Seq[BigDecimal]].isAssignableFrom(types(13)))
     assert(classOf[String].isAssignableFrom(types(14)))
     assert(classOf[java.lang.Float].isAssignableFrom(types(15)))
     assert(classOf[java.lang.Short].isAssignableFrom(types(16)))
-    assert(classOf[Seq[BigDecimal]].isAssignableFrom(types(17)))
+    assert(classOf[scala.collection.Seq[BigDecimal]].isAssignableFrom(types(17)))
     assert(rows(0).getString(0).equals("hello"))
     assert(rows(0).getInt(1) == 42)
     assert(rows(0).getDouble(2) == 1.25)