Add some simple tests on HashPMap

compiler/testData/codegen/boxWithStdlib/hashPMap/empty.kt

@@ -0,0 +1,22 @@
+import kotlin.reflect.jvm.internal.pcollections.HashPMap
+import kotlin.test.*
+
+fun box(): String {
+    val map = HashPMap.empty<String, Any>()!!
+
+    assertEquals(0, map.size())
+
+    assertFalse(map.containsKey(""))
+    assertFalse(map.containsKey("abacaba"))
+    assertEquals(null, map[""])
+    assertEquals(null, map["lol"])
+
+    // Check that doesn't create a new map
+    assertEquals(map, map.minus(""))
+
+    // Check that all empty()s are equal
+    val other = HashPMap.empty<String, Any>()!!
+    assertEquals(map, other)
+
+    return "OK"
+}