Fix "-Yrangepos" disappearing from Test / scalacOptions

Fixes https://github.com/sbt/sbt/issues/5934

In #5886 adpi2 reported that Test / scalacOptions could have unwanted duplicated flags from Compile / scalacOptions. So #5887 added

```diff
+    // remove duplicated semanticdbOptions
+    scalacOptions --= (Compile / semanticdbOptions).value
```

Notice that it's subtracting (Compile / semanticdbOptions).value regardless of the semanticdbEnabled value. If semanticdbEnabled is set to true I am guessing that it would all work out, but when it's false, it's just subtracting "-Yrangepos".

This fixes that by checking for semanticdbEnabled.

main/src/main/scala/sbt/plugins/SemanticdbPlugin.scala

@@ -70,9 +70,11 @@ object SemanticdbPlugin extends AutoPlugin {
       targetRootOptions(scalaVersion.value, semanticdbTargetRoot.value),
     scalacOptions --= Def.settingDyn {
       val config = configuration.value
-      Def.setting {
-        semanticdbOptions.?.all(ancestorConfigs(config)).value.flatten.flatten
-      }
+      val enabled = semanticdbEnabled.value
+      if (enabled)
+        Def.setting {
+          semanticdbOptions.?.all(ancestorConfigs(config)).value.flatten.flatten
+        } else Def.setting { Nil }
     }.value,
     scalacOptions ++= {
       if (semanticdbEnabled.value)

sbt/src/sbt-test/tests/munit/build.sbt

@@ -0,0 +1,10 @@
+ThisBuild / scalaVersion := "2.12.12"
+
+lazy val munit = "org.scalameta" %% "munit" % "0.7.22"
+
+lazy val root = (project in file("."))
+  .settings(
+    Compile / scalacOptions += "-Yrangepos",
+    libraryDependencies += munit % Test,
+    testFrameworks += new TestFramework("munit.Framework"),
+  )

sbt/src/sbt-test/tests/munit/src/test/scala/ClueSuite.scala

@@ -0,0 +1,40 @@
+package testpkg
+
+import munit._
+
+class ClueSuite extends FunSuite {
+  val x = 42
+  val y = 32
+  checkPrint(
+    "clues",
+    clues(x, y),
+    """|Clues {
+       |  x: Int = 42
+       |  y: Int = 32
+       |}
+       |""".stripMargin
+  )
+
+  val z: List[Int] = List(1)
+  checkPrint(
+    "list",
+    clues(z),
+    """|Clues {
+       |  z: List[Int] = List(
+       |    1
+       |  )
+       |}
+       |""".stripMargin
+  )
+
+  def checkPrint(
+      options: TestOptions,
+      clues: Clues,
+      expected: String
+  )(implicit loc: Location): Unit = {
+    test(options) {
+      val obtained = munitPrint(clues)
+      assertNoDiff(obtained, expected)
+    }
+  }
+}

sbt/src/sbt-test/tests/munit/test

@@ -0,0 +1 @@
+> test