Add test based on python code.

https://github.com/tensorflow/addons/blob/master/tensorflow_addons/activations/tests/softshrink_test.py

Related-To: Kotlin#170

api/src/test/kotlin/org/jetbrains/kotlinx/dl/api/core/activation/SoftShrinkActivationTest.kt

@@ -0,0 +1,24 @@
+package org.jetbrains.kotlinx.dl.api.core.activation
+
+import org.junit.jupiter.api.Test
+
+
+internal class SoftShrinkActivationTest : ActivationTest() {
+    @Test
+    fun defaultBoundaries() {
+        val input = floatArrayOf(-2.0f, -1.0f, 0.0f, 1.0f, 2.0f)
+        val actual = floatArrayOf(0f, 0f, 0f, 0f, 0f)
+        val expected = floatArrayOf(-1.5f, -0.5f, 0.0f, 0.5f, 1.5f)
+
+        assertActivationFunction(SoftShrinkActivation(), input, actual, expected)
+    }
+
+    @Test
+    fun explicitBoundaries() {
+        val input = floatArrayOf(-2.0f, -1.0f, 0.0f, 1.0f, 2.0f)
+        val actual = floatArrayOf(0f, 0f, 0f, 0f, 0f)
+        val expected = floatArrayOf(-1.0f, 0.0f, 0.0f, 0.0f, 1.0f)
+
+        assertActivationFunction(SoftShrinkActivation(lower = -1.0f, upper = 1.0f), input, actual, expected)
+    }
+}