Make libavb_host_unittest work when run outside //external/avb

The AVB unittests contain many relative path references to test
dependencies, like test data and the avbtool script. These dependencies
are already copied into the test directory through a data dependency in
the Soong build rule, but the test previously relied on being run with
PWD being correct. Now we set PWD explicitly in test setup.

Bug: None
Test: 'atest libavb_host_unittest' in Android source root (outside
//external/avb), all tests passed

Change-Id: Ic4318b17d3e72ef85bcc42518bebb15ab423dcb3
diff --git a/Android.bp b/Android.bp
index 20ade16..c4492cf 100644
--- a/Android.bp
+++ b/Android.bp
@@ -197,6 +197,7 @@
         "libgtest_host",
     ],
     shared_libs: [
+        "libbase",
         "libchrome",
         "libcrypto",
     ],