Disable two meta tests on Windows

The appveyor has problem with the tests:

- timeout: unknown error
- stackoverflow: VM crashes due to memory quota

Both tests succeed on local Windows machine.

Author: liufengyun

compiler/test/dotty/tools/vulpix/VulpixUnitTests.scala

@@ -65,6 +65,7 @@ class VulpixUnitTests extends ParallelTesting {
     compileFile("tests/vulpix-tests/unit/runDiffOutput1.scala", defaultOptions).expectFailure.checkRuns()
 
   @Test def runStackOverflow: Unit =
+    if (!scala.util.isWin)
     compileFile("tests/vulpix-tests/unit/stackOverflow.scala", defaultOptions).expectFailure.checkRuns()
 
   @Test def runOutRedirects: Unit =
@@ -90,7 +91,7 @@ class VulpixUnitTests extends ParallelTesting {
       case ae: AssertionError => assertTrue(ae.getMessage.contains("java compilation failed"))
     }
 
-  @Test def runTimeout: Unit = {
+  @Test def runTimeout: Unit = if (!scala.util.isWin) {
     val fileName = s"tests/vulpix-tests/unit/timeout.scala"
     try {
       compileFile(fileName, defaultOptions).checkRuns()