Duplicate tests and docs related to unittest.bzl from bazel_skylib to rules_testing in anticipation of their being deleted from bazel_skylib.

[copybara-service[bot]]

Duplicate tests and docs related to unittest.bzl from bazel_skylib to rules_testing in anticipation of their being deleted from bazel_skylib.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/rules_testing/pull/100 from bazelbuild:lberki-update-stardoc c78f47179dbeb6ae088c5aef68e5a1566750e1cc

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