build.sc: remove `testOnly` (#843)

* build.sc: remove `testOnly`

mill 0.98 added its own `testOnly`. If we keep our version, we can't compile build.sc.
Some people in the WeChat tutorial group have already encountered this issue.

* Pin mill version to 0.9.8

https://github.com/OpenXiangShan/XiangShan/pull/843#issuecomment-868669873

.mill-version

-0.9.6
+0.9.8

build.sc

@@ -101,10 +101,6 @@ object XiangShan extends CommonModule with SbtModule {
     def testFrameworks = Seq(
       "org.scalatest.tools.Framework"
     )
-
-    def testOnly(args: String*) = T.command {
-      super.runMain("org.scalatest.tools.Runner", args: _*)
-    }
   }
 
 }