Adds support for configuring TEST_MAPPING output in
update_crate_tests.py by defining a test_mapping_config.json.
The initial option that can be set is whether a test should be a
postsubmit test instead of a presubmit test.
Bug: 229727993
Test: update_crate_tests.py with test_mapping_config.json produces
postsubmit tests
Change-Id: I2f5a336c1af12630cc5df9d2c32ab63ac6099af8