BCR presubmit: support specify a test module

[meteorcloudy]

Context: https://groups.google.com/a/bazel.build/g/external-deps/c/GcV1cpRd-Ls/m/1UU9Ez_DDQAJ

In the presubmit.yml file, allow BCR modules to specify a test module (with bcr_test_module, module_path and its task configurations) to verify the basic functionalities of the checked-in module. The top level of task configurations are only used to verify the build targets.

#1300 makes the configuration file simpler.

Example (meteorcloudy/bazel-central-registry#7):

matrix:
  platform: ["centos7", "debian10", "macos", "ubuntu2004", "windows"]

tasks:
  verify_targets:
    name: "Verify build targets"
    platform: ${{ platform }}
    build_targets:
    - '@protobuf//:protobuf'
    - '@protobuf//:protobuf_lite'
    - '@protobuf//:protoc'
    - '@protobuf//:test_messages_proto2_proto_cc'

bcr_test_module:
  module_path: "examples"
  matrix:
    platform: ["centos7", "debian10", "macos", "ubuntu2004", "windows"]
  tasks:
    run_test_module:
      name: "Run test module"
      platform: ${{ platform }}
      build_targets:
      - "//..."

[meteorcloudy]

/cc @fmeum @alexeagle What do you think of this?

[fmeum]

Looks good to me. I like that this now has the same format as the top-level test config, just one level deeper in.

[meteorcloudy]

This is now implemented, @fmeum feel free to try it out and give us feedback!

[fmeum]

I am giving it a try in bazelbuild/bazel-central-registry#61