These tests are slow and can timeout, this diff increase the number of concurrent jobs to 4 and 8, which may help avoid timeouts.
These parameters are also configured differently in [many other tests](https://github.com/search?q=repo%3Agrpc%2Fgrpc+inner_jobs&type=code).
```
2024-03-10 07:00:56,453 START: run_tests_c_macos_dbg_native
2024-03-10 07:06:44,335 PASSED: run_tests_c_macos_dbg_native [time=347.9sec, retries=0:0]
2024-03-10 07:06:44,335 START: run_tests_c_macos_opt_native
2024-03-10 07:12:54,717 PASSED: run_tests_c_macos_opt_native [time=370.4sec, retries=0:0]
2024-03-10 07:12:54,717 START: run_tests_c++_macos_dbg_native
2024-03-10 09:19:00,785 PASSED: run_tests_c++_macos_dbg_native [time=7566.1sec, retries=0:0]
2024-03-10 09:19:00,788 START: run_tests_c++_macos_opt_native
ERROR: Aborting VM command due to timeout of 14400 seconds
```
Closes#36190
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36190 from HannahShiSFB:basic-c-cpp-macos-jobs c026a6afbc
PiperOrigin-RevId: 621306368
599b50228a