1. Move most of the generic stuff into BasePlatformConfig.
2. Make a GCB platform config that sets WORKSPACE and PROJECT_SRC_PATH so users don't have to.
3. Make a skeleton prow platform_config for Mitchel to fill out.
4. Make users explicitly specify the CFL_PLATFORM so we can pick CI environments
* [cifuzz] Create validate method on BaseConfig
Use it to validate that either OSS_FUZZ_PROJECT_NAME
or BUILD_INTEGRATION_PATH is set.
Also use it to validate that workspace is set
(rather than duplicate code).
Add tests.
* Use env var hack to bypass valdiation
* fix
* fix
* fmt
* fix
* tmp
* fix
1. Moving the _create_config and create_config functions from build_fuzzers_test.py and run_fuzzers_test.py into test_helpers.py (which is now part of cifuzz instead of infra) and share code between them.
2. Rename artifacts_dir to crashes_dir in run_fuzzers.py. "artifacts" is ambiguous.
3. Make some small changes to pytest.ini to improve debugging.