Fix AssemblyTests on Windows (#3288)

Windows has an env var called `PATHEXT` which is usually something like
this: `.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC`
Basically a list of extensions considered as executable.  
So the file in Mill's case needs to be a `.bat` file.

Issue analyzed and fixed by @sake92 

Fix #3283

Pull request: #3288
---------

Co-authored-by: Sakib Hadziavdic <sakib@sake.ba>

.config/mill-version

@@ -1 +1 @@
-0.11.7-17-17dea9
+0.11.8

scalalib/test/src/mill/scalalib/AssemblyTests.scala

@@ -1,5 +1,6 @@
 package mill.scalalib
 
+import scala.util.Properties
 import mill._
 import mill.api.Result
 import mill.eval.Evaluator
@@ -125,7 +126,14 @@ object AssemblyTests extends TestSuite {
         test("small") {
           workspaceTest(TestCase) { eval =>
             val Right((res, _)) = eval(TestCase.exe.small.assembly)
-            runAssembly(res.path, TestCase.millSourcePath, checkExe = true)
+            val originalPath = res.path
+            val resolvedPath =
+              if (Properties.isWin) {
+                val winPath = originalPath / os.up / s"${originalPath.last}.bat"
+                os.copy(originalPath, winPath)
+                winPath
+              } else originalPath
+            runAssembly(resolvedPath, TestCase.millSourcePath, checkExe = true)
           }
         }
         test("large-should-fail") {