Fix runGradingTest on windows

Fix for
https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/issues/9,
classpath are separated by ";" instead of ":" on Windows (because ":" is
needed to write "C:\..."), the solution is to use `File.pathSeparator`
which will choose the correct character for the current OS.

project/StudentTasks.scala

@@ -53,7 +53,7 @@ object StudentTasks extends AutoPlugin {
       throw new MessageOnlyException(s"Could not find tests JarFile: $testSuiteJar")
     }
 
-    val classPath = s"${(Test / dependencyClasspath).value.map(_.data).mkString(":")}:$testSuiteJar"
+    val classPath = s"${(Test / dependencyClasspath).value.map(_.data).mkString(File.pathSeparator)}${File.pathSeparator}$testSuiteJar"
     val junitProcess =
       Fork.java.fork(
         ForkOptions(),