Use a test helper executable to open files

In the test, we used a trick to open files from the shell, the pass the
FDs to fd_server. It will not work for directories.

This change replaces the trick with a test helper executable, which
supports opening directories as FD, in order to make the future test
cases that involved directory FD possible.

Bug: 203251769
Test: AuthFsHostTest
Change-Id: I77fdfa463f168bce99d1fccc91d0739bbc6ba00a
diff --git a/authfs/tests/AndroidTest.xml b/authfs/tests/AndroidTest.xml
index 6100ab9..9deab5b 100644
--- a/authfs/tests/AndroidTest.xml
+++ b/authfs/tests/AndroidTest.xml
@@ -31,6 +31,11 @@
     <target_preparer class="com.android.tradefed.targetprep.PushFilePreparer">
         <option name="cleanup" value="true" />
         <option name="abort-on-push-failure" value="true" />
+
+        <!-- Test executable -->
+        <option name="push-file" key="open_then_run" value="/data/local/tmp/authfs/open_then_run" />
+
+        <!-- Test data files -->
         <option name="push-file" key="cert.der" value="/data/local/tmp/authfs/cert.der" />
         <option name="push-file" key="input.4m" value="/data/local/tmp/authfs/input.4m" />
         <option name="push-file" key="input.4k1" value="/data/local/tmp/authfs/input.4k1" />