c2a: Do not assume test_mapping.json exists

update_crate_tests would previously assume test_mapping.json existed
when trying to generate a TEST_MAPPING update. This made updating
mappings for the majority of existing crates fail.

Bug: 233924440
Test: Regenerate all Rust TEST_MAPPINGs
Change-Id: I4d9b906f4db233ee0223a3035c1b63bf1ca681f4
1 file changed