Add tests for conv_linker_config

to cover the recent features.
- conv_linker_config proto with empty input
- conv_linker_config proto with existing output
- conv_linker_config proto with --append
- conv_linker_config proto with --force

Bug: n/a
Test: conv_linker_config_test
Change-Id: I0de79b6e05c2608e0e2f30dfbf04d8289672f362