Merge pull request #4906 from jtattermusch/runtest_more_cleanup

tools/dockerfile/grpc_sanity/Dockerfile

tools/dockerfile/grpc_tests_multilang_x64/Dockerfile

tools/dockerfile/grpc_tests_multilang_x86/Dockerfile

tools/jenkins/docker_run_tests.sh

tools/run_tests/run_tests.py

tools/run_tests/sanity/check_cache_mk.sh

tools/run_tests/sanity/check_sources_and_headers.py

tools/run_tests/sanity/check_submodules.sh

tools/run_tests/sanity/sanity_tests.yaml