bugfix: Try to fix Mill flaky test

My guess is that the test failed because it takes a while for the build server to connect. Let's see if this helps.

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

@@ -1,7 +1,10 @@
 package tests.mill
 
+import java.util.concurrent.TimeUnit
+
 import scala.concurrent.duration.Duration
 
+import scala.meta.internal.metals.MetalsEnrichments._
 import scala.meta.internal.metals.ServerCommands
 import scala.meta.internal.metals.{BuildInfo => V}
 
@@ -40,6 +43,8 @@ class MillServerCodeLensSuite
       _ <- server.initialize()
       _ <- server.initialized()
       _ <- server.executeCommand(ServerCommands.GenerateBspConfig)
+      _ <- server.server.indexingPromise.future
+        .withTimeout(2, TimeUnit.MINUTES)
       _ <- server.didOpen("MillMinimal/src/Main.scala")
       _ <- server.didSave("MillMinimal/src/Main.scala")(identity)
       _ = assertNoDiagnostics()