Add a simple test for running built-in fix rules alongside scalafix ones

Author: Gedochao

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

@@ -18,4 +18,53 @@ abstract class FixTestDefinitions
       s"--enable-scalafix=${enableScalafix.toString}",
       s"--enable-built-in-rules=${enableBuiltIn.toString}"
     )
+
+  test("built-in + scalafix rules") {
+    val mainFileName  = "Main.scala"
+    val unusedValName = "unused"
+    val directive1    = "//> using dep \"com.lihaoyi::os-lib:0.11.3\""
+    val directive2    = "//> using dep \"com.lihaoyi::pprint:0.9.0\""
+    val mergedDirective1And2 =
+      "using dependency \"com.lihaoyi::os-lib:0.11.3\" \"com.lihaoyi::pprint:0.9.0\""
+    val directive3 =
+      if (actualScalaVersion.startsWith("2")) "//> using options \"-Xlint:unused\""
+      else "//> using options \"-Wunused:all\""
+    TestInputs(
+      os.rel / "Foo.scala" ->
+        s"""$directive1
+           |object Foo {
+           |  def hello: String = "hello"
+           |}
+           |""".stripMargin,
+      os.rel / "Bar.scala" ->
+        s"""$directive2
+           |object Bar {
+           |  def world: String = "world"
+           |}
+           |""".stripMargin,
+      os.rel / mainFileName ->
+        s"""$directive3
+           |object Main {
+           |  def main(args: Array[String]): Unit = {
+           |    val unused = "unused"
+           |    pprint.pprintln(Foo.hello + Bar.world)
+           |    pprint.pprintln(os.pwd)
+           |  }
+           |}
+           |""".stripMargin,
+      os.rel / scalafixConfFileName ->
+        """rules = [
+          |  RemoveUnused
+          |]
+          |""".stripMargin
+    ).fromRoot { root =>
+      os.proc(TestUtil.cli, "fix", ".", extraOptions, "--power").call(cwd = root)
+      val projectFileContents = os.read(root / projectFileName)
+      expect(projectFileContents.contains(mergedDirective1And2))
+      expect(projectFileContents.contains(directive3))
+      val mainFileContents = os.read(root / mainFileName)
+      expect(!mainFileContents.contains(unusedValName))
+      os.proc(TestUtil.cli, "compile", ".", extraOptions).call(cwd = root)
+    }
+  }
 }