Add regression test for rendering explain message

Author: lwronski

modules/integration/src/test/scala/scala/cli/integration/CompileTestsDefault.scala

@@ -1,3 +1,23 @@
 package scala.cli.integration
 
-class CompileTestsDefault extends CompileTestDefinitions(scalaVersionOpt = None)
+import com.eed3si9n.expecty.Expecty.expect
+class CompileTestsDefault extends CompileTestDefinitions(scalaVersionOpt = None) {
+  test("render explain message") {
+    val fileName = "Hello.scala"
+    val inputs = TestInputs(
+      os.rel / fileName -> // should be dump to 3.3.1 after release
+        s"""//> using scala "3.3.1-RC1-bin-20230203-3ef1e73-NIGHTLY"
+           |//> using options "--explain"
+           |
+           |class A
+           |val i: Int = A()
+           |""".stripMargin
+    )
+    inputs.fromRoot { root =>
+      val out = os.proc(TestUtil.cli, "compile", extraOptions, fileName)
+        .call(cwd = root, check = false, mergeErrIntoOut = true).out.trim()
+
+      expect(out.contains("Explanation"))
+    }
+  }
+}