Run the TestAllFiles tests as part of Gradle

I accidentally excluded these in r1887643. They should not be excluded.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1887681 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/build.gradle b/build.gradle
index 8d0823c..eb7dff2 100644
--- a/build.gradle
+++ b/build.gradle
@@ -399,7 +399,6 @@
 
     test {
         // exclude these from the normal test-run
-        exclude '**/TestAllFiles.class'
         exclude '**/*FileHandler.class'
     }