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