reproducing the error.

m2/pom.xml

@@ -14,6 +14,13 @@
         <dependency>
             <groupId>org.scala-lang</groupId>
             <artifactId>scala-library</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.scalatest</groupId>
+            <artifactId>scalatest_2.12</artifactId>
+            <version>3.0.5</version>
+            <scope>test</scope>
         </dependency>
     </dependencies>
 </project>

m2/src/main/java/m2/M2Java.java

@@ -0,0 +1,7 @@
+package m2;
+
+public class M2Java {
+    public static void main(String[] args) {
+        System.out.println("hello, Maven, Scala, Java " + System.getProperty("java.specification.version"));
+    }
+}

m2/src/test/scala/m2/TestSpec.scala

@@ -0,0 +1,12 @@
+package m2
+
+import org.scalatest.FunSuite
+import org.scalatest.Matchers.convertToAnyShouldWrapper
+
+class TestSpec extends FunSuite {
+
+  test("x"){
+    List(1) should ===(List(1))
+  }
+
+}

pom.xml

@@ -24,11 +24,20 @@
         <plugin>
           <groupId>net.alchim31.maven</groupId>
           <artifactId>scala-maven-plugin</artifactId>
-          <version>3.2.2</version>
+          <version>3.3.1</version>
           <executions>
             <execution>
+              <id>scala-compile-first</id>
+              <phase>process-resources</phase>
               <goals>
+                <goal>add-source</goal>
                 <goal>compile</goal>
+              </goals>
+            </execution>
+            <execution>
+              <id>scala-test-compile</id>
+              <phase>process-test-resources</phase>
+              <goals>
                 <goal>testCompile</goal>
               </goals>
             </execution>