Ensure a Future is passed back to utest in all cases

tests/shared/src/test/scala/coursier/test/CentralTests.scala

@@ -63,7 +63,7 @@ abstract class CentralTests extends TestSuite {
     extraRepos: Seq[Repository] = Nil,
     configuration: String = "",
     profiles: Option[Set[String]] = None
-  ) =
+  ): Future[Unit] =
     async {
       val attrPathPart =
         if (module.attributes.isEmpty)
@@ -722,6 +722,8 @@ abstract class CentralTests extends TestSuite {
               val urls = artifacts.map(_.url).toSet
               assert(urls == expectedTarGzArtifactUrls)
             }
+          else
+            Future.successful(())
         }
         * - {
           withArtifacts(mod, version, "tar.gz", attributes = Attributes("tar.gz", "bin"), classifierOpt = Some("bin"), transitive = true) { artifacts =>
@@ -740,6 +742,8 @@ abstract class CentralTests extends TestSuite {
               val urls = artifacts.map(_.url).toSet
               assert(urls == expectedZipArtifactUrls)
             }
+          else
+            Future.successful(())
         }
         * - {
           withArtifacts(mod, version, "zip", attributes = Attributes("zip", "bin"), classifierOpt = Some("bin"), transitive = true) { artifacts =>
@@ -838,6 +842,8 @@ abstract class CentralTests extends TestSuite {
       * - {
         if (isActualCentral) // doesn't work via proxies, which don't list all the upstream available versions
           resolutionCheck(mod, ver)
+        else
+          Future.successful(())
       }
     }
 
@@ -848,6 +854,8 @@ abstract class CentralTests extends TestSuite {
       * - {
         if (isActualCentral) // if false, the tests rely on things straight from Central, which can be updated sometimes…
           resolutionCheck(mod, ver)
+        else
+          Future.successful(())
       }
     }
 
@@ -873,6 +881,8 @@ abstract class CentralTests extends TestSuite {
             assert(mainArtifactOpt.nonEmpty)
             assert(mainArtifactOpt.forall(_.isOptional))
           }
+        else
+          Future.successful(())
       }
 
       * - withArtifacts(mod, ver, "jar", optional = false) { artifacts =>
@@ -895,6 +905,8 @@ abstract class CentralTests extends TestSuite {
             )
             assert(urls == expectedUrls)
           }
+        else
+          Future.successful(())
       }
     }
 
@@ -969,6 +981,8 @@ abstract class CentralTests extends TestSuite {
 
             assert(expectedUrls.forall(urls))
           }
+        else
+          Future.successful(())
       }
     }
   }