MicrodroidHostTest: mkdir test dir @Before test run

Otherwise #testPathToBinaryIsRejected would fail to create files in the
dir.

Bug: 268184703
Test: reboot CF -> atest #testPathToBinaryIsRejected
Change-Id: Ifeb4a03eccefa7d9a84009b5db65508cfda21452
diff --git a/tests/hostside/helper/java/com/android/microdroid/test/host/MicrodroidHostTestCaseBase.java b/tests/hostside/helper/java/com/android/microdroid/test/host/MicrodroidHostTestCaseBase.java
index 20a6045..a7f7906 100644
--- a/tests/hostside/helper/java/com/android/microdroid/test/host/MicrodroidHostTestCaseBase.java
+++ b/tests/hostside/helper/java/com/android/microdroid/test/host/MicrodroidHostTestCaseBase.java
@@ -67,6 +67,8 @@
 
         // remove any leftover files under test root
         android.tryRun("rm", "-rf", TEST_ROOT + "*");
+
+        android.tryRun("mkdir " + TEST_ROOT);
     }
 
     public static void cleanUpVirtualizationTestSetup(ITestDevice androidDevice)