can: cache DBC parsing to speed up car interface tests

[logesh45]

Performance

Cuts test_car_interfaces.py runtime from ~127 s to ~57.19s (≈2.2× faster). The slowest test drops from ~11 s to ~1.66 s (that's 7x faster), this must meeting the target of ≤0.2 s avg and <1 s max per car.

Changes

• DBC caching: Added a class-level cache keyed by absolute file path so each DBC file is parsed only once per process. Subsequent CANParser instantiations reuse the cached instance.
• CANParser fast path: Early-return in CANParser.update when called with an empty frame list, avoiding needless buffer clearing.

Verification

Ran the test suite before and after the change:

pytest --durations=10 selfdrive/car/tests/test_car_interfaces.py