Add simple test about DefaultSource class

DefaultSource should have default constructor for
executors to be able to create it.

src/test/scala/cognite/spark/v1/DefaultSourceClassTest.scala

@@ -0,0 +1,24 @@
+package cognite.spark.v1
+
+import org.scalatest.{FlatSpec, Matchers}
+
+
+class DefaultSourceClassTest extends FlatSpec with Matchers {
+  it should "have default constructor" in {
+    // Data source should be constructable with no arguments and
+    // any parameters should be parsed within specific methods.
+    // Implicit parameters are also not allowed.
+    // Otherwise spark executors will fail to create it.
+    val res = new DefaultSource()
+    res shouldBe a[DefaultSource]
+  }
+
+  it should "really have a no-argument constructor" in {
+    // let's also check via reflection to rule out varargs option like A(String... s)
+    // getConstructor() will only pick up no-args version
+    // getConstructor(String[].class) can be used to select varargs version
+    // reflection also makes sure no scala implicits are needed
+    val res = classOf[DefaultSource].getConstructor().newInstance()
+    res shouldBe a[DefaultSource]
+  }
+}