Fix broken fs_config_generator tests

These tests were failing at TOT. Also add a
python_test_host target so the tests will be
run on CI in the future.

Bug: 203436762
Test: m libc, prebuilts/build-tools/linux-x86/bin/py2-cmd build/make/tools/fs_config/test_fs_config_generator.py
Change-Id: I6dda841023a2f5e76d59360d08626fc1a8842ffe