added other test for sort grouped df to make sure it's fixed

Author: Jolanrensen

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/sortGroupedDataframe.kt

@@ -0,0 +1,72 @@
+package org.jetbrains.kotlinx.dataframe.api
+
+import io.kotest.matchers.shouldBe
+import org.jetbrains.kotlinx.dataframe.DataFrame
+import org.jetbrains.kotlinx.dataframe.alsoDebug
+import org.jetbrains.kotlinx.dataframe.io.read
+import org.junit.Test
+
+class SortGroupedDataframeTests {
+
+    @Test
+    fun `Sorted grouped iris dataset`() {
+        val irisData = DataFrame.read("src/test/resources/irisDataset.csv")
+        irisData.alsoDebug()
+
+        irisData.groupBy("variety").let {
+            it.sortBy("petal.length").toString() shouldBe
+                it.sortBy { it["petal.length"] }.toString()
+        }
+    }
+
+    enum class State {
+        Idle, Productive, Maintenance
+    }
+
+    @Test
+    fun test4() {
+        class Event(val toolId: String, val state: State, val timestamp: Long)
+
+        val tool1 = "tool_1"
+        val tool2 = "tool_2"
+        val tool3 = "tool_3"
+
+        val events = listOf(
+            Event(tool1, State.Idle, 0),
+            Event(tool1, State.Productive, 5),
+            Event(tool2, State.Idle, 0),
+            Event(tool2, State.Maintenance, 10),
+            Event(tool2, State.Idle, 20),
+            Event(tool3, State.Idle, 0),
+            Event(tool3, State.Productive, 25),
+        ).toDataFrame()
+
+        val lastTimestamp = events.maxOf { getValue<Long>("timestamp") }
+        val groupBy = events
+            .groupBy("toolId")
+            .sortBy("timestamp")
+            .add("stateDuration") {
+                (next()?.getValue("timestamp") ?: lastTimestamp) - getValue<Long>("timestamp")
+            }
+
+        groupBy.toDataFrame().alsoDebug()
+        groupBy.schema().print()
+        groupBy.keys.print()
+        groupBy.keys[0].print()
+
+        val df1 = groupBy.updateGroups {
+            val missingValues = State.values().asList().toDataFrame {
+                "state" from { it }
+            }
+
+            val df = it
+                .fullJoin(missingValues, "state")
+                .fillNulls("stateDuration")
+                .with { 100L }
+
+            df.groupBy("state").sumFor("stateDuration")
+        }
+
+        df1.toDataFrame().alsoDebug().isNotEmpty() shouldBe true
+    }
+}