refactor: ensure we're testing the connection with Mill

When I opened com-lihaoyi/mill#2123 it made me
wonder why we didn't hit on this in our tests, but then I realized we
were actually never testing the initialization with Mill server. This
adds some more tests to ensure we're not only testing the generation of
the BSP config file, but also connection to the build server.

tests/slow/src/test/scala/tests/mill/MillServerSuite.scala

@@ -1,5 +1,7 @@
 package tests.mill
 
+import java.util.concurrent.TimeUnit
+
 import scala.meta.internal.builds.MillBuildTool
 import scala.meta.internal.builds.MillDigest
 import scala.meta.internal.metals.Messages
@@ -13,7 +15,7 @@ import tests.MillBuildLayout
 import tests.MillServerInitializer
 
 /**
- * Basic suite to ensure that a connection to sbt server can be made.
+ * Basic suite to ensure that a connection to a Mill server can be made.
  */
 class MillServerSuite
     extends BaseImportSuite("mill-server", MillServerInitializer) {
@@ -49,25 +51,36 @@ class MillServerSuite
     }
   }
 
-  test("generate") {
-    def millBspConfig = workspace.resolve(".bsp/mill-bsp.json")
-    cleanWorkspace()
-    writeLayout(MillBuildLayout("", V.scala213, supportedBspVersion))
-    for {
-      _ <- server.initialize()
-      _ <- server.initialized()
-      _ = assertNoDiff(
-        client.workspaceMessageRequests,
-        // Project has no .bloop directory so user is asked to "import via bloop"
-        // since bloop is still the default
-        importBuildMessage,
-      )
-      _ = client.messageRequests.clear() // restart
-      _ = assert(!millBspConfig.exists)
-      // At this point, we want to use mill-bsp server, so create the mill-bsp.json file.
-      _ <- server.executeCommand(ServerCommands.GenerateBspConfig)
-    } yield {
-      assert(millBspConfig.exists)
+  val versionsToTest: List[String] =
+    List("0.10.0", "0.10.8", supportedBspVersion)
+
+  versionsToTest.foreach(testGenerationAndConnection)
+
+  private def testGenerationAndConnection(version: String) = {
+    test(s"generate-and-connect-$version") {
+      def millBspConfig = workspace.resolve(".bsp/mill-bsp.json")
+      cleanWorkspace()
+      writeLayout(MillBuildLayout("", V.scala213, version))
+      for {
+        _ <- server.initialize()
+        _ <- server.initialized()
+        _ = assertNoDiff(
+          client.workspaceMessageRequests,
+          // Project has no .bloop directory so user is asked to "import via bloop"
+          // since bloop is still the default
+          importBuildMessage,
+        )
+        _ = client.messageRequests.clear() // restart
+        _ = assert(!millBspConfig.exists)
+        // At this point, we want to use mill-bsp server, so create the mill-bsp.json file.
+        _ <- server.executeCommand(ServerCommands.GenerateBspConfig)
+        // We need to wait a bit just to ensure the connection is made
+        _ <- server.waitFor(TimeUnit.SECONDS.toMillis(1))
+      } yield {
+        assert(millBspConfig.exists)
+        server.assertBuildServerConnection()
+      }
     }
+
   }
 }