Add tests for REPL completions related to #5895

Author: anatoliykmetyuk

compiler/test/dotty/tools/repl/TabcompleteTests.scala

@@ -124,4 +124,12 @@ class TabcompleteTests extends ReplTest {
     val comp = tabComplete("???.")
     assertEquals(Nil, comp)
   }
+
+  @Test def opaqueTypeCompletions = fromInitialState { implicit s =>
+    assertEquals(List("Predef"), tabComplete("opaque type T = Pre"))
+  }
+
+  @Test def moduleCompletion = fromInitialState { implicit s =>
+    assertEquals(List("Predef"), tabComplete("object Foo { type T = Pre"))
+  }
 }